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 https://gitlab.com/gitlab-org/gitlab/-/issues/331776 for more
information.