lib: add thread subqueries.

lib: add thread subqueries.

This change allows queries of the form

 thread:{from:me} and thread:{from:jian} and not thread:{from:dave}

This is still somewhat brute-force, but it's a big improvement over
both the shell script solution and the previous proposal [1], because it
does not build the whole thread structure just generate a
query. A further potential optimization is to replace the calls to
notmuch with more specialized Xapian code; in particular it's not
likely that reading all of the message metadata is a win here.

[1]: id:20170820213240.20526-1-david@tethera.net