pgsql: Support an optional asynchronous commit mode, in which we don't

Log Message:
-----------
Support an optional asynchronous commit mode, in which we don't flush WAL
before reporting a transaction committed. Data consistency is still
guaranteed (unlike setting fsync = off), but a crash may lose the effects
of the last few transactions. Patch by Simon, some editorialization by Tom.

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.133 -> r1.134)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.133&r2=1.134)
wal.sgml (r1.44 -> r1.45)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/wal.sgml.diff?r1=1.44&r2=1.45)
pgsql/src/backend/access/transam:
README (r1.5 -> r1.6)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/README.diff?r1=1.5&r2=1.6)
clog.c (r1.42 -> r1.43)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/clog.c.diff?r1=1.42&r2=1.43)
multixact.c (r1.23 -> r1.24)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/multixact.c.diff?r1=1.23&r2=1.24)
slru.c (r1.40 -> r1.41)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/slru.c.diff?r1=1.40&r2=1.41)
subtrans.c (r1.18 -> r1.19)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/subtrans.c.diff?r1=1.18&r2=1.19)
transam.c (r1.69 -> r1.70)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/transam.c.diff?r1=1.69&r2=1.70)
twophase.c (r1.31 -> r1.32)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/twophase.c.diff?r1=1.31&r2=1.32)
xact.c (r1.245 -> r1.246)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.245&r2=1.246)
xlog.c (r1.275 -> r1.276)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.275&r2=1.276)
pgsql/src/backend/commands:
dbcommands.c (r1.196 -> r1.197)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c.diff?r1=1.196&r2=1.197)
tablespace.c (r1.48 -> r1.49)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablespace.c.diff?r1=1.48&r2=1.49)
vacuum.c (r1.353 -> r1.354)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c.diff?r1=1.353&r2=1.354)
pgsql/src/backend/utils/init:
flatfiles.c (r1.26 -> r1.27)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/flatfiles.c.diff?r1=1.26&r2=1.27)
pgsql/src/backend/utils/misc:
guc.c (r1.407 -> r1.408)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.407&r2=1.408)
postgresql.conf.sample (r1.221 -> r1.222)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.221&r2=1.222)
pgsql/src/backend/utils/time:
tqual.c (r1.102 -> r1.103)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/time/tqual.c.diff?r1=1.102&r2=1.103)
pgsql/src/include/access:
clog.h (r1.19 -> r1.20)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/clog.h.diff?r1=1.19&r2=1.20)
gist_private.h (r1.26 -> r1.27)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/gist_private.h.diff?r1=1.26&r2=1.27)
slru.h (r1.20 -> r1.21)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/slru.h.diff?r1=1.20&r2=1.21)
transam.h (r1.60 -> r1.61)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/transam.h.diff?r1=1.60&r2=1.61)
xact.h (r1.87 -> r1.88)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xact.h.diff?r1=1.87&r2=1.88)
xlog.h (r1.81 -> r1.82)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlog.h.diff?r1=1.81&r2=1.82)
xlogdefs.h (r1.17 -> r1.18)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlogdefs.h.diff?r1=1.17&r2=1.18)