Get rid of integer divide in FAST_PATH_REL_GROUP() macro

I noticed a while ago that the new fast-path locking code uses integer
division to figure out the  fast-path locking group slot.  To me, this
seems a bit unnecessary as FastPathLockGroupsPerBackend is always a
power-of-two value, so we can use bitwise-AND instead.

I don't think FAST_PATH_REL_GROUP() is in any particularly hot code
paths, but still, having the divide in there isn't sitting well with
me. Can we get rid of it?

I've attached a patch for that. I also adjusted the method used to
calculate FastPathLockGroupsPerBackend. Also, the Assert that was
going on at the end of the loop in InitializeFastPathLocks() looked a
little odd as it seems to be verifying something that the loop
condition was checking already. I thought it was better to check that
we end up with a power-of-two.

Please see the attached patch.

David