pgsql: Merge synchronous_replication setting into synchronous_commit.

Merge synchronous_replication setting into synchronous_commit.

This means one less thing to configure when setting up synchronous
replication, and also avoids some ambiguity around what the behavior
should be when the settings of these variables conflict.

Fujii Masao, with additional hacking by me.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/240067b3b0ff989d2b28f4f917f7958f2ec76b05

Modified Files
--------------
doc/src/sgml/config.sgml                      |   68 +++++++++----------------
doc/src/sgml/high-availability.sgml           |   48 +++++++----------
doc/src/sgml/release-9.1.sgml                 |    7 ++-
src/backend/access/transam/xact.c             |    5 +-
src/backend/postmaster/autovacuum.c           |    3 +-
src/backend/replication/syncrep.c             |    1 -
src/backend/utils/misc/guc.c                  |   43 ++++++++++------
src/backend/utils/misc/postgresql.conf.sample |    6 +--
src/include/access/xact.h                     |   11 +++-
src/include/replication/syncrep.h             |    3 +-
10 files changed, 91 insertions(+), 104 deletions(-)