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