Replace custom module for `libcluster` Postgres strategy with `libcluster_postgres`

[hauleth]

Chore

Describe the chore

Currently we are using custom module for libcluster's Postgres strategy, but it already got improved and extracted to separate library - https://hex.pm/packages/libcluster_postgres. We should use that instead of maintaining it on our own.

Additional context

It is Supabase project, but the external library is much more maintained and tested than our custom solution there.

[linear]

POOLER-146 Replace custom module for `libcluster` Postgres strategy with `libcluster_postgres`

[abc3]

If you want to fix #616 by switching to libcluster_postgres, it won’t help because it’s just a copy of supavisor's module and has the same potential buggy behavior.