enhance wraparound warnings

varsup.c has the following comment:

	/*
	 * We'll start complaining loudly when we get within 40M transactions of
	 * data loss.  This is kind of arbitrary, but if you let your gas gauge
	 * get down to 2% of full, would you be looking for the next gas station?
	 * We need to be fairly liberal about this number because there are lots
	 * of scenarios where most transactions are done by automatic clients that
	 * won't pay attention to warnings.  (No, we're not gonna make this
	 * configurable.  If you know enough to configure it, you know enough to
	 * not get in this kind of trouble in the first place.)
	 */

I don't know about you, but I start getting antsy around a quarter tank.
In any case, I'm told that even 40M transactions aren't enough time to
react these days.  Attached are a few patches to enhance the wraparound
warnings.

* 0001 adds a "percent remaining" detail message to the existing WARNING.
The idea is that "1.86% of transaction IDs" is both easier to understand
and better indicates urgency than "39985967 transactions".

* 0002 bumps the warning limit from 40M to 100M to give folks some more
time to react.

* 0003 adds an early warning system for when fewer than 500M transactions
remain.  This system sends a LOG only to the server log every 1M
transactions.  The hope is that this gets someone's attention sooner
without flooding the application and server log.

Thoughts?

-- 
nathan