Lag tracking for logical replication

Lag tracking is called for each commit, but we introduce
a pacing delay to ensure we don't swamp the lag tracker.

Author: Petr Jelinek, with minor pacing delay code from me

src/backend/replication/slotfuncs.c

@@ -133,7 +133,8 @@ pg_create_logical_replication_slot(PG_FUNCTION_ARGS)
 	 */
 	ctx = CreateInitDecodingContext(NameStr(*plugin), NIL,
 									false, /* do not build snapshot */
-									logical_read_local_xlog_page, NULL, NULL);
+									logical_read_local_xlog_page, NULL, NULL,
+									NULL);
 
 	/* build initial snapshot, might take a while */
 	DecodingContextFindStartpoint(ctx);