Try to avoid running with a full fsync request queue.

When we need to insert a new entry and the queue is full, compact the
entire queue in the hopes of making room for the new entry.  Doing this
on every insertion might worsen contention on BgWriterCommLock, but
when the queue it's full, it's far better than allowing the backend to
perform its own fsync, per testing by Greg Smith as reported in
http://archives.postgresql.org/pgsql-hackers/2011-01/msg02665.php

Original idea from Greg Smith.  Patch by me.  Review by Chris Browne
and Greg Smith