Add an initial implementation of a notmuch_thread_t object.

We've now got a new notmuch_query_search_threads and a
notmuch_threads_result_t iterator. The thread object itself
doesn't do much yet, (just allows one to get the thread_id),
but that's at least enough to see that "notmuch search" is
actually doing something now, (since it has been converted
to print thread IDs instead of message IDs).

And maybe that's all we need. Getting the messages belonging
to a thread is as simple as a notmuch_query_search_messages
with a string of "thread:<thread-id>".

Though it would be convenient to add notmuch_thread_get_messages
which could use the existing notmuch_message_results_t iterator.

Now we just need an implementation of "notmuch show" and we'll
have something somewhat usable.

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