WALWriter active during recovery

Currently, WALReceiver writes and fsyncs data it receives. Clearly,
while we are waiting for an fsync we aren't doing any other useful
work.

Following patch starts WALWriter during recovery and makes it
responsible for fsyncing data, allowing WALReceiver to progress other
useful actions.

At present this is a WIP patch, for code comments only. Don't bother
with anything other than code questions at this stage.

Implementation questions are

* How should we wake WALReceiver, since it waits on a poll(). Should
we use SIGUSR1, which is already used for latch waits, or another
signal?

* Should we introduce some pacing delays if the WALreceiver gets too
far ahead of apply?

* Other questions you may have?

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