pg_rewind support

It's pretty common to promote a replica server of a database cluster when performing maintenance, i.e. maintenance is first done on the replica and after the maintenance is done and replica is fully synced up the old master is shut down and replica promoted. Typically we want to make the old master a replica after maintenance has been performed there, but PG doesn't allow master to switch roles to a replica too easily.

pg_rewind can be used for this, but running it is a bit cumbersome and access to WAL archive is required after it has been run, so it would be useful if PGHoard could take care of running pg_rewind and then setting up the old master as a replica, with proper recovery.conf pointing to PGHoard's wal archive and an optional primary_conninfo line connecting it to the new master.