Fixed #22634 -- Changed Session and SessionStore to be easily extendable

Introduced abstract base model for Session, and properties enabling the developer to override session model class used by session store and the session store class used by the model.

Allowed to override KEY_PREFIX via the cache_key_prefix property.
This can go an "example" subsection. See https://github.com/django/django/commit/9aeb917dad2c3d4c2830f38d3bfc63699e0ba837#diff-661c56868d52efa1e939067487b1af6dR319 for example.