pgstat: delayed write of stats file

Per some earlier discussion, here is an attempt at implementing a
"delayed write" of the pgstats file, to decrease the write activity on
that file.

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