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/logical/logicalfuncs.c

@@ -253,7 +253,7 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 									options,
 									logical_read_local_xlog_page,
 									LogicalOutputPrepareWrite,
-									LogicalOutputWrite);
+									LogicalOutputWrite, NULL);
 
 		MemoryContextSwitchTo(oldcontext);