shm_mq: Reduce spinlock usage.

Previously, mq_bytes_read and mq_bytes_written were protected by the
spinlock, but that turns out to cause pretty serious spinlock
contention on queries which send many tuples through a Gather or
Gather Merge node.  This patches changes things so that we instead
read and write those values using 8-byte atomics.  Since mq_bytes_read
can only be changed by the receiver and mq_bytes_written can only be
changed by the sender, the only purpose of the spinlock is to prevent
reads and writes of these values from being torn on platforms where
8-byte memory access is not atomic, making the conversion fairly
straightforward.

Testing shows that this produces some slowdown if we're using emulated
64-bit atomics, but since they should be available on any platform
where performance is a primary concern, that seems OK.  It's faster,
sometimes a lot faster, on platforms where such atomics are available.

Patch by me, reviewed by Andres Freund, who also suggested the
design.  Also tested by Rafia Sabih.

Discussion: http://postgr.es/m/CA+TgmoYuK0XXxmUNTFT9TSNiBtWnRwasBcHHRCOK9iYmDLQVPg@mail.gmail.com