Win32 latch implementation revisited

It just occurred to me that the Windows latch implementation goes
through a lot of trouble to dynamically assign the shared Windows event
handles to the latches in OwnLatch, but there's really no reason why
they can't be statically assigned in InitSharedLatch instead. We have to
allocate the same amount of event handles anyway.

That makes the implementation a lot simpler, eliminating the shared
memory block dedicated to latches altogether, and all the related
bookkeeping. We no longer need NumSharedLatches() function anymore
either, each InitSharedLatch call can allocate a new event handle directly.

It was the separation of InitSharedLatch and OwnLatch steps that made
this possible, which is probably why this didn't occur to me earlier.
Separating those steps was definitely a good move.

Unless someone sees a problem with this, I'll commit the attached patch
to do the simplification.

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com