This refactors Gitlab::Database::LoadBalancing::LoadBalancer so it can
support the use of multiple databases (each with their own load
balancer) in the future.

Instead of directly depending on ActiveRecord::Base, LoadBalancer now
takes a ActiveRecord model. It will then scope its work to that model,
including the data that it caches in RequestStore. This ensures two
different LoadBalancer instances won't conflict with each other.

The new setup of retrieving connections is such that we can ensure we
always have the right connection (e.g. a writable connection when we
need one).

See https://gitlab.com/gitlab-org/gitlab/-/issues/331776 for more
information.
Co-Authored-By: Thong Kuah <tkuah@gitlab.com>