pgsql: Add a txn_start column to pg_stat_activity.

Log Message:
-----------
Add a txn_start column to pg_stat_activity. This makes it easier to
identify long-running transactions. Since we already need to record
the transaction-start time (e.g. for now()), we don't need any
additional system calls to report this information.

Catversion bumped, initdb required.

Modified Files:
--------------
pgsql/doc/src/sgml:
monitoring.sgml (r1.41 -> r1.42)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/monitoring.sgml.diff?r1=1.41&r2=1.42)
pgsql/src/backend/access/transam:
xact.c (r1.229 -> r1.230)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.229&r2=1.230)
pgsql/src/backend/catalog:
system_views.sql (r1.32 -> r1.33)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/system_views.sql.diff?r1=1.32&r2=1.33)
pgsql/src/backend/postmaster:
pgstat.c (r1.140 -> r1.141)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.140&r2=1.141)
pgsql/src/backend/utils/adt:
pgstatfuncs.c (r1.34 -> r1.35)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/pgstatfuncs.c.diff?r1=1.34&r2=1.35)
pgsql/src/include:
pgstat.h (r1.50 -> r1.51)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pgstat.h.diff?r1=1.50&r2=1.51)
pgsql/src/include/catalog:
catversion.h (r1.360 -> r1.361)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.360&r2=1.361)
pg_proc.h (r1.429 -> r1.430)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h.diff?r1=1.429&r2=1.430)
pgsql/src/test/regress/expected:
rules.out (r1.121 -> r1.122)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/rules.out.diff?r1=1.121&r2=1.122)