Added settings.SESSION_SERIALIZER which is the import path of a serializer
to use for sessions.
Thanks apollo13, carljm, shaib, akaariai, charettes, and dstufft for reviews.
Backport of b0ce6fe656 from master
Correctly calculate the ``aggregate_start`` offset from loaded fields,
if any are deferred, instead of ``self.query.select`` which includes all
fields on the model.
Also made some PEP 8 fixes.