Change ReplicationSlotPersistentData's "synced" member to a bool.

Note that this doesn't require bumping SLOT_VERSION because we
require sizeof(bool) == 1, thanks to commit 97525bc5c8.

Overight in commit ddd5f4f54a.

Discussion: Ranier Vilela <ranier.vf@gmail.com>

src/include/replication/slot.h

@@ -134,7 +134,7 @@ typedef struct ReplicationSlotPersistentData
 	/*
 	 * Was this slot synchronized from the primary server?
 	 */
-	char		synced;
+	bool		synced;
 
 	/*
 	 * Is this a failover slot (sync candidate for standbys)? Only relevant