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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

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