Bring back the insert_thread_id function.

We deleted this in favor of our fancy new thread_ids iterator
from the message object. But one of the previous callers of
insert_thread_id isn't using notmuch_message_t yet. I made
the mistake of thinking I could just call g_hash_table_insert
directly, but the problem was that nobody was splitting
up the thread_id string at its commas.

So with this, we were inserting bogus comma-separated IDs
into the hash table, so thread_id values were ballooning
out of control. Should be much better now.