[notmuch] reverse iterators

[Ruben Pollan]

Adds support to reverse iteration on messages, threads and tags. To revew and
think if makes sense to include them on notmuch or wait until they have a real
use.

The patch 3 "Move the logic of threads iterator out of 'valid'" is just a 
reorganization of the code, I think it makes sense to include it in notmuch even
if we decide that the reverse iterators are not included.