Add support for temporary replication slots

This allows creating temporary replication slots that are removed
automatically at the end of the session or on error.

From: Petr Jelinek <petr.jelinek@2ndquadrant.com>

src/backend/storage/lmgr/proc.c

@@ -810,6 +810,9 @@ ProcKill(int code, Datum arg)
 	if (MyReplicationSlot != NULL)
 		ReplicationSlotRelease();
 
+	/* Also cleanup all the temporary slots. */
+	ReplicationSlotCleanup();
+
 	/*
 	 * Detach from any lock group of which we are a member.  If the leader
 	 * exist before all other group members, it's PGPROC will remain allocated