Make `LoadBalancer` to re-use very similar connection

What does this MR do and why?

This allows to have two LBs configured, each of them using a different set of replicas, but the same primary connection to handle transaction visibility.

Challenges:

• this does not yet have any Feature Flag behavior
• this makes me a small thrills as it works only for model.connection, but if anything would do model.retrieve_connection this would cause a problems as a new connection would be open

Related to: #341451 (closed), based on !72694 (closed).

Validation

Requires decomposed CI configured in GDK/GCK.

1. Disabled:

# GITLAB_LOAD_BALANCING_REUSE_PRIMARY_ci unset

$ bin/rails console

[4] pry(main)> Ci::InstanceVariable.last
  ...
  Ci::InstanceVariable Load (14.9ms)  SELECT "ci_instance_variables".* FROM "ci_instance_variables" ORDER BY "ci_instance_variables"."id" DESC LIMIT 1 # application:console,db_config_name:ci_replica,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/marginalia-1.10.0/lib/marginalia/comment.rb:25:in `block in construct_comment'
=> db_config_name:ci_replica expected

[6] pry(main)> Ci::InstanceVariable.create!(key: 'aa', value: 'bb')
  ...
  TRANSACTION (0.4ms)  COMMIT # application:console,db_config_name:ci,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/marginalia-1.10.0/lib/marginalia/comment.rb:25:in `block in construct_comment'
=> db_config_name:ci expected

2. Enabled

export GITLAB_LOAD_BALANCING_REUSE_PRIMARY_ci=main

$ bin/rails console

[1] pry(main)> Ci::InstanceVariable.last
  Ci::InstanceVariable Load (0.8ms)  SELECT "ci_instance_variables".* FROM "ci_instance_variables" ORDER BY "ci_instance_variables"."id" DESC LIMIT 1 # application:console,db_config_name:ci_replica,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/marginalia-1.10.0/lib/marginalia/comment.rb:25:in `block in construct_comment'
=> db_config_name:ci_replica expected

[3] pry(main)> Ci::InstanceVariable.create!(key: 'aa2', value: 'bb')
  ...
  TRANSACTION (0.5ms)  COMMIT # application:console,db_config_name:main,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/marginalia-1.10.0/lib/marginalia/comment.rb:25:in `block in construct_comment'
=> db_config_name:main expected

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.