message properties, round 2

[David Bremner]

This is still marked RFC, but is a lot closer to production ready than
the last round. In particular it is now integrated with the term
caching machinery in message.cc. The main implementation cost is the
creation of string-map.c to provide associative arrays of strings,
with iterators (basically mimicing the Xapian::TermIterators). I'm not
sure if these can replace some of the uses of gmime hash tables in the
CLI (or if that would be a good idea), but if so that code could move to util,
and probably lose the _notmuch_ prefix.