Rewrite the contacts-cache backend in c++
Store the contacts as metadata in the xapian database, rather than in a
separate file.
Update the Store to deal with this.
which, if true, means that the contact was seen in a message where at least
one of the addresses in the recipients field was 'my' address (this is
decided when in mu-store-write.cc). using this, we can exclude mailing list
posts.