Reducing ClogControlLock contention

ClogControlLock contention is high at commit time. This appears to be due
to the fact that ClogControlLock is acquired in Exclusive mode prior to
marking commit, which then gets starved by backends running
TransactionIdGetStatus().

Proposal for improving this is to acquire the ClogControlLock in Shared
mode, if possible.

This is safe because people checking visibility of an xid must always run
TransactionIdIsInProgress() first to avoid race conditions, which will
always return true for the transaction we are currently committing. As a
result, we never get concurrent access to the same bits in clog, which
would require a barrier.

Two concurrent writers might access the same word concurrently, so we
protect against that with a new CommitLock. We could partition that by
pageno also, if needed.

--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
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