pgsql: Add new replication mode synchronous_commit = 'write'.

Add new replication mode synchronous_commit = 'write'.
Replication occurs only to memory on standby, not to disk,
so provides additional performance if user wishes to
reduce durability level slightly. Adds concept of multiple
independent sync rep queues.

Fujii Masao and Simon Riggs

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/443b4821f1649bc617c5ce1f6f3ffc65842a8930

Modified Files
--------------
doc/src/sgml/config.sgml                    |   18 +++-
doc/src/sgml/high-availability.sgml         |   16 +++-
src/backend/replication/syncrep.c           |  112 ++++++++++++++++++---------
src/backend/replication/walsender.c         |    3 +-
src/backend/utils/misc/guc.c                |    5 +-
src/include/access/xact.h                   |    1 +
src/include/replication/syncrep.h           |   13 +++-
src/include/replication/walsender_private.h |    8 +-
8 files changed, 124 insertions(+), 52 deletions(-)