Make `LoadBalancer` to configure pool sizes of all classes

What does this MR do and why?

Safer variant of !72706 (closed). I will still get back to !72706 (closed), but for now this is way safer to rollout to production as it unifies when configuration is set.

This changes database_config.rb and moves connection pool size configuration to Load Balancer.

This is enabled by default with GITLAB_LB_CONFIGURE_CONNECTION=true as this is in general a safe change. The FF is just in-case.

Related to:

• #333411 (closed)
• alternative take !72706 (closed)

How to set up and validate locally (requires decomposition enabled in GDK/GCK)

Default state:

unset GITLAB_LB_CONFIGURE_CONNECTION # enabled by default

bin/rails console

[2] pry(main)> ActiveRecord::Base.connection_db_config.pool
=> 11 # in general value > 10
[3] pry(main)> Ci::CiDatabaseRecord.connection_db_config.pool
=> 11 # this is properly configured and have value > 10

Disabled (old behavior):

export GITLAB_LB_CONFIGURE_CONNECTION=1

bin/rails console

[1] pry(main)> ActiveRecord::Base.connection_db_config.pool
=> 11
[2] pry(main)> Ci::CiDatabaseRecord.connection_db_config.pool
=> 5 # this was not configured prior to this change, so 5 represents default

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