test infrastructure for new dump/restore

[david at tethera.net]

This implements an old suggestion of Mark's to get wacky message-ids
into the database directly without relying on underdefined behaviour
of the gmime parser; the previous effort relied on gmime passing
literally through message-ids not delimitted according RFC.

Also compared to the previous version
(id:"1326591624-15493-10-git-send-email-david at tethera.net"), this now
uses valid UTF-8 text, rather than just ascii, although it is a bit
biased towards ascii because most of the characters that cause
problems are there.

There is a fair amount of code here, but I hope the generation of
random messages may be more useful in the future.

The high level goal here is to (re)-introduce a hex-encoding based
dump-restore that can pass this roundtrip test, and probably some
batch tagging facility that shares code.

If people don't mind things broken up into mini-series (without
obvious gain in new features) like this, I'll probably post the
hex-encoding infrastructure next.