pgsql: Take the statistics collector out of the loop for monitoring

Log Message:
-----------
Take the statistics collector out of the loop for monitoring backends'
current commands; instead, store current-status information in shared
memory. This substantially reduces the overhead of stats_command_string
and also ensures that pg_stat_activity is fully up to date at all times.
Per my recent proposal.

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.65 -> r1.66)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.65&r2=1.66)
monitoring.sgml (r1.33 -> r1.34)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/monitoring.sgml.diff?r1=1.33&r2=1.34)
pgsql/src/backend/postmaster:
pgstat.c (r1.128 -> r1.129)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.128&r2=1.129)
postmaster.c (r1.486 -> r1.487)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.486&r2=1.487)
pgsql/src/backend/storage/ipc:
ipci.c (r1.83 -> r1.84)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/ipci.c.diff?r1=1.83&r2=1.84)
procarray.c (r1.11 -> r1.12)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/procarray.c.diff?r1=1.11&r2=1.12)
pgsql/src/backend/utils/adt:
pgstatfuncs.c (r1.29 -> r1.30)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/pgstatfuncs.c.diff?r1=1.29&r2=1.30)
pgsql/src/backend/utils/misc:
guc.c (r1.321 -> r1.322)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.321&r2=1.322)
postgresql.conf.sample (r1.179 -> r1.180)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.179&r2=1.180)
pgsql/src/include:
pgstat.h (r1.46 -> r1.47)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pgstat.h.diff?r1=1.46&r2=1.47)
pgsql/src/include/storage:
procarray.h (r1.8 -> r1.9)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/procarray.h.diff?r1=1.8&r2=1.9)