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