starsessions 2.0

[alex-oleshkevich]

What I'd like to see in the next release:

1. automatic session loading (probably via loop.run_until_complete)
2. move serializers to middleware (now they are in backends)
3. require explicit backend and remove secret_key from middleware arguments
4. ability to encrypt session contents, this is mandatory when using shared Redis instances
5. regenerate_id should be a utility function, not a part of Session class because this makes mypy sad: request.session.regenerate_id() vs new_id = regenerate_id(request.session)
6. cleanup Session class interface: consider removing all non-mapping methods from it, they are not compatible with Starlette's session interface
7. change Backend.write signature to async def write(self, session_id: str, data: typing.Dict) -> str, session_id to be required

Anything else?