Sync 9.5 version of access/transam/parallel.c with HEAD.

Sync 9.5 version of access/transam/parallel.c with HEAD.

This back-patches commit a5fe473ad (notably, marking ParallelMessagePending
as volatile, which is not particularly optional).  I also back-patched some
previous cosmetic changes to remove unnecessary diffs between the two
branches.  I'm unsure how much of this code is actually reachable in 9.5,
but to the extent that it is reachable, it needs to be maintained, and
minimizing cross-branch diffs will make that easier.