pgsql: Reduce checkpoints and WAL traffic on low activity database serv

Reduce checkpoints and WAL traffic on low activity database server
Previously, we skipped a checkpoint if no WAL had been written since
last checkpoint, though this does not appear in user documentation.
As of now, we skip a checkpoint until we have written at least one
enough WAL to switch the next WAL file. This greatly reduces the
level of activity and number of WAL messages generated by a very
low activity server. This is safe because the purpose of a checkpoint
is to act as a starting place for a recovery, in case of crash.
This patch maintains minimal WAL volume for replay in case of crash,
thus maintaining very low crash recovery time.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/18fb9d8d21a28caddb72c7ffbdd7b96d52ff9724

Modified Files
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src/backend/access/transam/xlog.c |   28 +++++++++++++++-------------
1 files changed, 15 insertions(+), 13 deletions(-)