make work with cascading replication

[plockaby]

The current replication_slots check doesn't work with cascading replication. This makes it work.

[machack666]

So this should work going back to 9.2 (or whichever version introduced cascaded replication)?

[plockaby]

Correct that cascading replication was introduced in 9.2. I'm pretty certain that it will work on everything going back to 9.2 From the 9.2 docs:

They can be retrieved using pg_current_xlog_location on the primary and the pg_last_xlog_receive_location on the standby, respectively.

That line changed like this on 10:

These locations can be retrieved using pg_current_wal_lsn on the primary and pg_last_wal_receive_lsn on the standby, respectively.