Isolate `Gitlab::Database::LoadBalancing` code into `gitlab-database-load_balancing` gem
Problem
The Gitlab::Database::LoadBalancing
is a code with a significant technical debt,
as it implements our "specific" way to handle database load balancing replicas
that is vastly different in a behavior to Rails.
Proposal
As part of first step of improving this part of the codebase we should aim to isolate it, to make significant better handle on the set of dependencies required by this feature.
The best solution is to use GitLab Monorepo Gems &10869 and store all Load Balancing specific code in a gem to identify exactly minimal set of dependencies required by this feature, and plan how to migrate towards more Rails specific replicas.