Switch to matrix-nio library

[ulope]

Abstract

We're currently using the (so far) official matrix-python-sdk (Python package matrix-client) Matrix client library.
This so far doesn't support E2E encryption and probably won't in the foreseeable future.

In private conversation Mathew from Matrix pointed out that matrix-nio is the new preferred library and fully supports E2E encryption (via python-olm).

Motivation

We need to prevent MitM / front running attacks (#473).
E2E encryption is one way of providing this protection (#3893, #4105).

Tasks

• Investigate the amount of work needed to switch to matrix-nio
• Plan implementation
• Implement

Backwards Compatibility

This is purely an internal change.
Possibly some abstraction leakage may require some changes in the services.

[hackaugusto]

Please, also consider removing the dependency cycle among RaidenService and the Transport.