Track individual lb in Gitlab::Database::LoadBalancing::Session

What does this MR do and why?

This MR updates Gitlab::Database::LoadBalancing::Session to track use_primary and performed_writes on a per-db basis rather than using a single boolean. The method calls to specify database are mainly found in Sticking and ConnectionProxy classes.

This is preparation work for configuring per-db data consistency in sidekiq workers.

gitlab-com/gl-infra/scalability#3805

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