pgsql: Delay write of pg_stats file to once every five minutes, during

Log Message:
-----------
Delay write of pg_stats file to once every five minutes, during
shutdown, or when requested by a backend:

It changes so the file is only written once every 5 minutes (changeable
of course, I just picked something) instead of once every half second.
It's still written when the stats collector shuts down, just as before.
And it is now also written on backend request. A backend requests a
rewrite by simply sending a special stats message. It operates on the
assumption that the backends aren't actually going to read the
statistics file very often, compared to how frequent it's written today.

Magnus Hagander

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        pgstat.c (r1.123 -> r1.124)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.123&r2=1.124)
    pgsql/src/include:
        pgstat.h (r1.43 -> r1.44)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pgstat.h.diff?r1=1.43&r2=1.44)