GitHub Importer: Handle `Cells::TransactionRecord::Error` claim conflict when creating Placeholder Users
## Summary
The GitHub Importer raises a `Cells::TransactionRecord::Error` (gRPC `ALREADY_EXISTS` / code 6) when attempting to create Placeholder Users.
Note: the error is not limited to Github Importer. It affects all importers
## Actual behavior
A `Cells::TransactionRecord::Error` is raised with the following conflict details:
```
GRPC::AlreadyExists: 6:claim conflict: 3 record(s):
{bucket_type=ROUTES bucket_value="libhe_placeholder_16e65vk"}
{bucket_type=USERNAMES bucket_value="libhe_placeholder_16e65vk"}
{bucket_type=EMAILS bucket_value="libhe_placeholder_16e65vk@noreply.gitlab.com"}
```
The error originates in `app/models/cells/transaction_record.rb` during the `before_committed!` callback, triggered via the database load balancer write path.
## Sentry reference
Sentry Issue: [GITLABCOM-D9PQ](https://new-sentry.gitlab.net/organizations/gitlab/issues/3563978/?referrer=gitlab_integration)
## Stack trace
```
GRPC::AlreadyExists: 6:claim conflict: 3 record(s): {bucket_type=ROUTES bucket_value="libhe_placeholder_16e65vk"}, {bucket_type=USERNAMES bucket_value="libhe_placeholder_16e65vk"}, {bucket_type=EMAILS bucket_value="libhe_placeholder_16e65vk@noreply.gitlab.com"}.
from grpc/generic/active_call.rb:29:in `check_status'
from grpc/generic/active_call.rb:189:in `attach_status_results_and_complete_call'
from grpc/generic/active_call.rb:383:in `request_response'
from grpc/generic/client_stub.rb:180:in `block in request_response'
from grpc/generic/interceptors.rb:178:in `block (2 levels) in intercept!'
from grpc/generic/interceptors.rb:178:in `block (2 levels) in intercept!'
from grpc/generic/interceptors.rb:178:in `block (2 levels) in intercept!'
from grpc/generic/interceptors.rb:181:in `block in intercept!'
from labkit/correlation/grpc/client_interceptor.rb:20:in `request_response'
from grpc/generic/interceptors.rb:175:in `intercept!'
from grpc/generic/interceptors.rb:177:in `block in intercept!'
from gitlab/cells/topology_service/metadata_client.rb:16:in `request_response'
from grpc/generic/interceptors.rb:175:in `intercept!'
from grpc/generic/interceptors.rb:177:in `block in intercept!'
from lib/gitlab/topology_service_client/concurrency_limit_interceptor.rb:30:in `block in request_response'
from lib/gitlab/topology_service_client/concurrency_limit_interceptor.rb:53:in `block in intercept'
from lib/gitlab/topology_service_client/concurrency_limit_interceptor.rb:74:in `with_concurrency_limit'
from lib/gitlab/topology_service_client/concurrency_limit_interceptor.rb:53:in `intercept'
from lib/gitlab/topology_service_client/concurrency_limit_interceptor.rb:30:in `request_response'
from grpc/generic/interceptors.rb:175:in `intercept!'
from grpc/generic/interceptors.rb:177:in `block in intercept!'
from lib/gitlab/topology_service_client/metrics_interceptor.rb:22:in `block in request_response'
from lib/gitlab/topology_service_client/metrics_interceptor.rb:223:in `with_metrics_recording'
from lib/gitlab/topology_service_client/metrics_interceptor.rb:21:in `request_response'
from grpc/generic/interceptors.rb:175:in `intercept!'
from grpc/generic/client_stub.rb:179:in `request_response'
from grpc/generic/service.rb:171:in `block (3 levels) in rpc_stub_class'
from lib/gitlab/topology_service_client/claim_service.rb:15:in `begin_update'
from app/models/cells/outstanding_lease.rb:15:in `create_from_request!'
from app/models/cells/transaction_record.rb:107:in `block in before_committed!'
from retriable.rb:61:in `block in retriable'
from <internal:numeric>:237:in `times'
from retriable.rb:56:in `retriable'
from app/models/cells/transaction_record.rb:104:in `before_committed!'
from active_record/connection_adapters/abstract/transaction.rb:285:in `block in before_commit_records'
from active_record/connection_adapters/abstract/transaction.rb:344:in `run_action_on_records'
from active_record/connection_adapters/abstract/transaction.rb:284:in `before_commit_records'
from active_record/connection_adapters/abstract/transaction.rb:588:in `block in commit_transaction'
from active_support/concurrency/null_lock.rb:9:in `synchronize'
from active_record/connection_adapters/abstract/transaction.rb:584:in `commit_transaction'
from active_record/connection_adapters/abstract/transaction.rb:628:in `block in within_new_transaction'
from active_support/concurrency/null_lock.rb:9:in `synchronize'
from active_record/connection_adapters/abstract/transaction.rb:613:in `within_new_transaction'
from active_record/connection_adapters/abstract/database_statements.rb:361:in `transaction'
from lib/gitlab/database/load_balancing/connection_proxy.rb:127:in `public_send'
from lib/gitlab/database/load_balancing/connection_proxy.rb:127:in `block in write_using_load_balancer'
from lib/gitlab/database/load_balancing/load_balancer.rb:141:in `block in read_write'
from lib/gitlab/database/load_balancing/load_balancer.rb:264:in `retry_with_backoff'
from lib/gitlab/database/load_balancing/load_balancer.rb:131:in `read_write'
from lib/gitlab/database/load_balancing/connection_proxy.rb:126:in `write_using_load_balancer'
from lib/gitlab/database/load_balancing/connection_proxy.rb:78:in `transaction'
from active_record/transactions.rb:234:in `block in transaction'
from lib/gitlab/database/load_balancing/setup.rb:73:in `block in setup_connection_proxy'
from active_record/transactions.rb:233:in `transaction'
from lib/gitlab/database.rb:399:in `transaction'
from app/models/concerns/cross_database_modification.rb:82:in `transaction'
from lib/gitlab/import/source_user_mapper.rb:100:in `create_source_user_mapping'
from lib/gitlab/import/source_user_mapper.rb:91:in `block in create_source_user'
from lib/gitlab/exclusive_lease_helpers.rb:43:in `block in in_lock'
from lib/gitlab/exclusive_lease_helpers.rb:53:in `with_instrumentation'
from lib/gitlab/exclusive_lease_helpers.rb:42:in `in_lock'
from lib/gitlab/import/source_user_mapper.rb:83:in `create_source_user'
from lib/gitlab/import/source_user_mapper.rb:59:in `find_or_create_source_user'
from lib/gitlab/github_import/user_finder.rb:102:in `source_user'
from lib/gitlab/github_import/user_finder.rb:93:in `user_id_for'
from lib/gitlab/github_import/user_finder.rb:66:in `author_id_for'
```
issue
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