lib/thread-fp: factor out query expansion, rewrite in Xapian

lib/thread-fp: factor out query expansion, rewrite in Xapian

It will be convenient not to have to construct a notmuch query object
when parsing subqueries, so the commit rewrites the query
expansion (currently only used for thread:{} queries) using only
Xapian. As a bonus it seems about 15% faster in initial experiments.