Move DBModel._id -> DBModel.id

Move DBModel._id -> DBModel.id

We were refering to model._id in most of the code base as this is
what Mongo uses. However, each use of _id required a) fixup of queries:
e.g. what we did in our find() and find_one() functions moving all
'_id' to 'id'. It also required using AliasFields to make the ._id
attribute available. This all means lots of superfluous fixing and
transitioning in a SQL world.

It will also not work in the long run. Much newer code already refers
to the objects by model.id (e.g. in the oauth plugin), which will break
with Mongo. So let's be honest, rip out the _id mongoism and live with
.id as the one canonical way to address objects.

This commit modifies all users and providers of model._id to use
model.id instead. This patch works with or without Mongo removed first,
but will break Mongo usage (even more than before)

I have not bothered to fixup db.mongo.* and db.sql.convert
(which converts from Mongo to SQL)

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>