Replication slots are not supported in Aurora. Attempting to check
the lag results in the message:

```
ActiveRecord::StatementInvalid: PG::FeatureNotSupported: ERROR:
Replication slots are currently not supported in Aurora : SELECT
pg_xlog_location_diff(pg_current_xlog_insert_location(),
restart_lsn)::...
```

To avoid breaking support for background migrations in Aurora, we just
disable the check if we encounter this error.

This change also now checks whether there are any replication slots
present in the primary before checking the replication lag.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52176