Previously, the notmuch_messages_t object was a linked list built on
top of a linked-list node with the odd name of notmuch_message_list_t.
Now, we've got much more sane naming with notmuch_message_list_t being
a list built on a linked-list node named notmuch_message_node_t. And
now the public notmuch_messages_t object is a separate iterator based
on notmuch_message_node_t. This means the interfaces for the new
notmuch_message_list_t object are now made available to the library
internals.
The new object is simply a linked-list of notmuch_message_t objects,
(unlike the old object which contained a couple of Xapian iterators).
This works now by the query code immediately iterator over all results
and creating notmuch_message_t objects for them, (rather than waiting
to create the objects until the notmuch_messages_get call as we did
earlier).
The point of this change is to allow other instances of lists of
messages, (such as in notmuch_thread_t), that are not directly related
to Xapian search results.
