miscsearch: index UIDVALIDITY, use as startup cache

miscsearch: index UIDVALIDITY, use as startup cache

This brings -nntpd startup time down from ~35s to ~5s with 50K
inboxes.

Further improvements ought to be possible with deeper changes to
MiscIdx, since -mda having to load every inbox seems unreasonable;
but this general change is fairly unintrusive.