*index: don't try to index boolean terms >245 bytes long

*index: don't try to index boolean terms >245 bytes long

Xapian's flint, chert, and glass backends only support a maximum
term length of 245 bytes including the upper-case term prefix.
Thus we can't index things like insanely long Message-IDs in
References: or overly long pathnames or newsgroup names.

The work-in-progress honey backend still doesn't support
updates, yet, (only created from an existing glass DB), and I
doubt the limit will increase since excessive term lengths are
usually a mistake of mangled whitespace or a broken/spammy
client somewhere.