pithos: Improve test

Extend test for listing objects until a specific timestamp
to cover the case that an object is deleted after the provided timestamp.
pithos: PEP8 fix
pithos: Reset negative object sizes

Provide a database migration script that discovers
erroneously declared negative objects sizes and sets
them to 0.
pithos: Raise custom exception for invalid policies.

Raise 'InvalidPolicy' instead of 'ValuerError' exception
during account/container updates.
pithos: Change PithosBackendPool signature

PithosBackendPool constructor used to accept a number of keyword arguments
that it would pass them to the ModularBackend during the backend
instantiation.
Hence, this code required refactoring each time a new backend argument
was introduced.
This commit changes the PithosBackendPool to accept
a kwargs dictionary instead.