Fix transaction pollution in Shard.by_name

What does this MR do?

Per documentation in https://docs.gitlab.com/ee/development/sql.html#find_or_create_by-is-not-atomic

Spotted while reviewing https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23508

Currently, we only use Shard.by_name in one place, where it is not run inside a transaction. So it's OK. However, in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23508 , we start calling it from within a transaction.

In that case, the raising of ActiveRecord::RecordNotUnique pollutes the surrounding transaction. For Shard.by_name to be safe in all cases, we need it to happen within its own new transaction.

What are the relevant issue numbers?

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