lib: add function to get the number of threads matching a search

lib: add function to get the number of threads matching a search

Add function notmuch_query_count_threads() to get the number of threads
matching a search. This is done by performing a search and figuring out the
number of unique thread IDs in the matching messages, a significantly
heavier operation than notmuch_query_count_messages().

Signed-off-by: Jani Nikula <jani@nikula.org>