Assert(LWLockHeldByMeInMode(lock, LW_EXCLUSIVE))

Hi hackers,

Several times now when reading, debugging and writing code I've wished
that LWLockHeldByMe assertions specified the expected mode, especially
where exclusive locking is required.

What do you think about something like the attached?  See also an
example of use.  I will add this to the next commitfest.

--
Thomas Munro
http://www.enterprisedb.com