lib: Move message ID compression to _notmuch_message_create_for_message_id

lib: Move message ID compression to _notmuch_message_create_for_message_id

Previously, this was performed by notmuch_database_add_message.  This
happens to be the only caller currently (which is why this was safe),
but we're about to introduce more callers, and it makes more sense to
put responsibility for ID compression in the lower-level function
rather than requiring each caller to handle it.