Introduce changes to Session.cpp that allow to decrypt with session object alone

Summary:
This differential refactors CryptoModule and Session objects. The following changes are introduced:

1. Session object doesn't hold reference to CryptoModule anymore. This reference was just passed in constructor and never used in any method.
2. Session object can decrypt message independently.
3. CryptoModule can remove session from its internal map.

All changes above were necessary to enable notifications code to work with sessions objects only.

Test Plan: Text that encrypted notifs work correctly.

Reviewers: kamil, tomek, michal

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11233