In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68042 we found
that releasing primary connections can result in test suite errors. This
can happen when tests reuse a connection also used by Puma, but release
it instead of letting the Puma thread release it. Reproducing this is
difficult, but it happens often enough to be a problem.

Since Rails already releases model connections for us, we don't actually
need to release primary connections in the first place. See the
following for more information:

- https://tenderlovemaking.com/2011/10/20/connection-management-in-activerecord.html
- https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68042#note_672850193

This commit changes the load balancer code to not release primary
connections, with the exception of one case where this _is_ needed.

Changelog: fixed