pgsql: Add a function pg_stat_clear_snapshot() that discards any

Log Message:
-----------
Add a function pg_stat_clear_snapshot() that discards any statistics snapshot
already collected in the current transaction; this allows plpgsql functions to
watch for stats updates even though they are confined to a single transaction.
Use this instead of the previous kluge involving pg_stat_file() to wait for
the stats collector to update in the stats regression test.  Internally,
decouple storage of stats snapshots from transaction boundaries; they'll
now stick around until someone calls pgstat_clear_snapshot --- which xact.c
still does at transaction end, to maintain the previous behavior.  This makes
the logic a lot cleaner, at the price of a couple dozen cycles per transaction
exit.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        monitoring.sgml (r1.45 -> r1.46)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/monitoring.sgml.diff?r1=1.45&r2=1.46)
    pgsql/src/backend/access/transam:
        xact.c (r1.232 -> r1.233)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.232&r2=1.233)
    pgsql/src/backend/postmaster:
        pgstat.c (r1.144 -> r1.145)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.144&r2=1.145)
    pgsql/src/backend/utils/adt:
        pgstatfuncs.c (r1.37 -> r1.38)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/pgstatfuncs.c.diff?r1=1.37&r2=1.38)
    pgsql/src/include/catalog:
        catversion.h (r1.381 -> r1.382)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.381&r2=1.382)
        pg_proc.h (r1.442 -> r1.443)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h.diff?r1=1.442&r2=1.443)
    pgsql/src/include:
        pgstat.h (r1.52 -> r1.53)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pgstat.h.diff?r1=1.52&r2=1.53)
    pgsql/src/test/regress/expected:
        stats.out (r1.9 -> r1.10)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/stats.out.diff?r1=1.9&r2=1.10)
    pgsql/src/test/regress/sql:
        stats.sql (r1.7 -> r1.8)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/stats.sql.diff?r1=1.7&r2=1.8)