Eliminate divide in new fast-path locking code

c4d5cb71d2 adjusted the fast-path locking code to allow some
configuration of the number of fast-path locking slots via the
max_locks_per_transaction GUC.  In that commit the FAST_PATH_REL_GROUP()
macro used integer division to determine the fast-path locking group slot
to use for the lock.

The divisor in this case is always a power-of-two value.  Here we swap
out the divide by a bitwise-AND, which is a significantly faster
operation to perform.

In passing, adjust the code that's setting FastPathLockGroupsPerBackend
so that it's more clear that the value being set is a power-of-two.

Also, adjust some comments in the area which contained some magic
numbers.  It seems better to justify the 1024 upper limit in the
location where the #define is made instead of where it is used.

Author: David Rowley <drowleyml@gmail.com>
Reviewed-by: Tomas Vondra <tomas@vondra.me>
Discussion: https://postgr.es/m/CAApHDvodr3bcnpxcs7+k-3cFwYR0tP-BYhyd2PpDhe-bCx9i=g@mail.gmail.com

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