Verify replication slots upon startup

[jesperpedersen]

We should verify that backup_slot and wal_slot are valid on the server in question.

If not, then it is a FATAL and pgmoneta should be stopped.

[jesperpedersen]

F.ex.

postgres=# SELECT slot_name, slot_type FROM pg_replication_slots;
 slot_name | slot_type 
-----------+-----------
 repl      | physical
(1 row)

[Jubilee101]

Got it! I'll work on this. One problem we found constantly is missing WAL segments when there's no replication slot configured. Currently we do nothing when there is no replication slot configured. Should we make wal_slot a required configuration? Or can we create a replication slot automatically for user if there's none?

[jesperpedersen]

I added #160 for the first part, and #161 for the latter

[jesperpedersen]

Check should happen in https://github.com/pgmoneta/pgmoneta/blob/main/src/main.c#L1757

[Jubilee101]

Got it!