Flaky test: Release DB proxy's host when clearing configuration

Fix the flaky test in #325786 (comment 536276808)

When running the database load balancing tests in order, the second test fails.

bundle exec rspec ee/spec/lib/gitlab/database/load_balancing_spec.rb ee/spec/lib/gitlab/database/load_balancing/connection_proxy_spec.rb

$ bundle exec rspec ee/spec/lib/gitlab/database/load_balancing_spec.rb ee/spec/lib/gitlab/database/load_balancing/connection_proxy_spec.rb

All examples were filtered out; ignoring {:focus=>true}
....................................-- create_table(:load_balancing_test, {:force=>true})
   -> 0.0084s
.........................-- drop_table(:load_balancing_test, {:force=>true})
   -> 0.0025s
.............-- create_table(:connection_proxy_bulk_insert, {:force=>true})
   -> 0.0050s
-- drop_table(:connection_proxy_bulk_insert, {:force=>true})
   -> 0.0014s
F............

Failures:

  1) Gitlab::Database::LoadBalancing::ConnectionProxy.insert_all! inserts data in bulk
     Failure/Error:
       expect do
         model_class.insert_all! [
           { name: "item1" },
           { name: "item2" }
         ]
       end.to change { model_class.count }.by(2)

     ActiveRecord::StatementInvalid:
       PG::UndefinedTable: ERROR:  relation "connection_proxy_bulk_insert" does not exist
       LINE 1: SELECT COUNT(*) FROM "connection_proxy_bulk_insert" /*applic...
                                    ^
     # ./ee/lib/gitlab/database/load_balancing/connection_proxy.rb:90:in `block in read_using_load_balancer'
     # ./ee/lib/gitlab/database/load_balancing/load_balancer.rb:40:in `read'
     # ./ee/lib/gitlab/database/load_balancing/connection_proxy.rb:89:in `read_using_load_balancer'
     # ./ee/lib/gitlab/database/load_balancing/connection_proxy.rb:46:in `select_all'
     # ./ee/spec/lib/gitlab/database/load_balancing/connection_proxy_spec.rb:104:in `block (4 levels) in <main>'
     # ./ee/spec/lib/gitlab/database/load_balancing/connection_proxy_spec.rb:99:in `block (3 levels) in <main>'
     # ./spec/spec_helper.rb:365:in `block (3 levels) in <top (required)>'
     # ./spec/support/sidekiq_middleware.rb:9:in `with_sidekiq_server_middleware'
     # ./spec/spec_helper.rb:356:in `block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:350:in `block (2 levels) in <top (required)>'
     # ------------------
     # --- Caused by: ---
     # PG::UndefinedTable:
     #   ERROR:  relation "connection_proxy_bulk_insert" does not exist
     #   LINE 1: SELECT COUNT(*) FROM "connection_proxy_bulk_insert" /*applic...
     #                                ^
     #   ./ee/lib/gitlab/database/load_balancing/connection_proxy.rb:90:in `block in read_using_load_balancer'

Finished in 19.73 seconds (files took 46.22 seconds to load)
87 examples, 1 failure

Failed examples:

rspec ./ee/spec/lib/gitlab/database/load_balancing/connection_proxy_spec.rb:90 # Gitlab::Database::LoadBalancing::ConnectionProxy.insert_all! inserts data in bulk

Here are the root cause of the failure:

• The default configuration of the database cleaner is transaction, it means that all SQL queries inside a test are wrapped inside a big transaction, then rollback after a test is done. The transaction is fetched from ActiveRecord::Base.connection. The states and results of those queries are invisible to other connections, regardless they connect to the same DB in the same process.
• In ee/spec/lib/gitlab/database/load_balancing_spec.rb, as it's a integration test, the database load balancer is configured for real, with:

let(:hosts) { [ActiveRecord::Base.configurations["development"]['host']] }

before do
  subject.configure_proxy(::Gitlab::Database::LoadBalancing::ConnectionProxy.new(hosts))
end

The load balancer creates a new connection pool for each host. Unfortunately, when picking the host to fetch the connection from, the load balancer saves the host into the request store:

        def host
          RequestStore[CACHE_KEY] ||= @host_list.next
        end

• In the next test (ee/spec/lib/gitlab/database/load_balancing/connection_proxy_spec.rb), the load balancer returns a connection from the host generated by the previous test. That connection is different from the connection database_cleaner used to define the test table. Hence, the relation not exist .

The solution is straightforward: clean up and release the host when clearing DB load balancing configuration.