Improve HJDEBUG code a bit.

Commit 30d7ae3c introduced an HJDEBUG
stanza that probably didn't compile at the time, and definitely doesn't
compile now, because it refers to a nonexistent variable.  It doesn't seem
terribly useful anyway, so just get rid of it.

While I'm fooling with it, use %z modifier instead of the obsolete hack of
casting size_t to unsigned long, and include the HashJoinTable's address in
each printout so that it's possible to distinguish the activities of
multiple hashjoins occurring in one query.

Noted while trying to use HJDEBUG to investigate bug #13908.  Back-patch
to 9.5, because code that doesn't compile is certainly not very helpful.