python: Continuing message re-use fix

[Floris Bruynooghe]

Hi,

This builds on the patch by Anton Khirnov to fix the message re-use
that is possible when accessing messages from a thread.  I started
with just addressing my own comments on this patch, but evolved it
into switching the logic around and leave the normal Message object
untouched.  Instead I created a new OwnedMessage which is used by
the Thread which does not free itself on __del__().  I think this
is preferable because the other iterators, mainly Database.messages(),
do not allow retrieving messages more than once since the query object
is hidden from the API.

I've left the original commit in this patch series to not alter any
contributions.

Cheers,
Floris