Refactor Gitlab::Database to support multiple DBs

What does this MR do?

This refactors the module Gitlab::Database so it supports multiple databases. Logic tied to a connection is now scoped to instances of Gitlab::Dababase::Connection. For every database, an instance of this class is created and stored in Gitlab::Database::DATABASES.

The module Gitlab::Database still exposes various methods that forward the call to a corresponding instance of Database::Connection. These wrappers and their use will be changed in separate commits, as otherwise the diffs become far too large for anybody to make sense of.

See #331776 (closed) for more information.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
  • This is a backstage-only change
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.