This reduces unnecessary exposure of other headers through htup.h, which
is very widely included by many files.
I have chosen to move the function prototypes to the new file as well,
because that means htup.h no longer needs to include tupdesc.h. In
itself this doesn't have much effect in indirect inclusion of tupdesc.h
throughout the tree, because it's also required by execnodes.h; but it's
something to explore in the future, and it seemed best to do the htup.h
change now while I'm busy with it.
and add a note about why. This is not tremendously important right now,
probably, but it will get more urgent if NUM_BUFFER_PARTITIONS is increased
as much as proposed.
pointers, to ensure that compilers won't rearrange accesses to occur
while we're not holding the buffer header spinlock. It's probably
not necessary to mark volatile in every single place in bufmgr.c,
but better safe than sorry. Per trouble report from Kevin Grittner.
