Setup error context callback for transaction lock waits

With this in place, a session blocking behind another one because of
tuple locks will get a context line mentioning the relation name, tuple
TID, and operation being done on tuple.  For example:

LOG:  process 11367 still waiting for ShareLock on transaction 717 after 1000.108 ms
DETAIL:  Process holding the lock: 11366. Wait queue: 11367.
CONTEXT:  while updating tuple (0,2) in relation "foo"
STATEMENT:  UPDATE foo SET value = 3;

Most usefully, the new line is displayed by log entries due to
log_lock_waits, although of course it will be printed by any other log
message as well.

Author: Christian Kruse, some tweaks by Álvaro Herrera
Reviewed-by: Amit Kapila, Andres Freund, Tom Lane, Robert Haas

src/backend/access/nbtree/nbtinsert.c

@@ -168,7 +168,7 @@ top:
 		{
 			/* Have to wait for the other guy ... */
 			_bt_relbuf(rel, buf);
-			XactLockTableWait(xwait);
+			XactLockTableWait(xwait, rel, &itup->t_tid, XLTW_InsertIndex);
 			/* start over... */
 			_bt_freestack(stack);
 			goto top;