Reduced power consumption in WAL Writer process

Attached is patch for the WAL writer that removes its tight polling
loop (which probably doesn't get hit often in practice, as we just
sleep if wal_writer_delay is under a second), and, at least
potentially, reduces power consumption when idle by using a latch.

I will break all remaining power consumption work down into
per-auxiliary process patches. I think that this is appropriate - if
we hit a snag on one of the processes, there is no need to have that
hold up everything.

I've commented that we handle all expected signals, and therefore we
shouldn't worry about having timeout invalidated by signals, just as
with the archiver. Previously, we didn't even worry about Postmaster
death within the tight polling loop, presumably because
wal_writer_delay is typically small enough to avoid that being a
problem. I thought that WL_POSTMASTER_DEATH might be superfluous here,
but then again there is a codepath specifically for the case where
wal_writer_delay exceeds one second, so it is included in this initial
version.

Comments?

--
Peter Geoghegan       http://www.2ndQuadrant.com/
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