lib/query: Fix notmuch_threads_t to stream results rather than blocking.

lib/query: Fix notmuch_threads_t to stream results rather than blocking.

Previously, notmuch_query_search_threads would do all the work, so the
caller would block until all results were processed. Now, we do the
work as we go, as the caller iterates with notmuch_threads_next. This
means that once results start coming back from "notmuch search" they
just keep continually streaming.

There's still some initial blocking before the first results appear
because the notmuch_messages_t object has the same bug (for now).