Use local Unix domain socket connection for password sync

[mgomezch]

Instead of trying to use a superuser connection, the operator could exec psql in database pods to sync passwords, which would use a local connection through the PostgreSQL Unix domain socket. This would allow the operator to sync passwords even if the superuser secret is not consistent with a running cluster's state, which in turn would remove the need to preserve secrets upon cluster deletion, and simplify the process of restoring backups.

This may have a very low priority as the current workaround of not deleting secrets is effective.

[kupson]

That would be a possible solution for #524.