Group Commit

Enclosed patch implements Group Commit and also powersave mode for WALWriter.

XLogFlush() waits for WALWriter to run XLogBackgroundFlush(), which
flushes WAL and then wakes waiters. Uses same concepts and similar
code to sync rep.

Purpose is to provide consistent WAL writes, even when WALInsertLock
contended. Currently no "off" option, thinking is that the overhead of
doing this is relatively low and so it can be "always on" - exactly as
it is for sync rep.

WALWriter now has variable wakeups, so wal_writer_delay is removed.
Commit_delay and Commit_siblings are now superfluous and are also removed.

Works, but needs discussion in some areas, docs and possibly tuning
first, so this is more of a quicky than a slow, comfortable patch.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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