Evaluate Matrix Olm e2e encryption

[err508]

Motivation

We need transport e2e encryption, see #473

Specification

We will use Matrix's e2e encryption. To be answered:

• Current status of implementation/maintenance in the python client
• Performance
• Cross Server compatibility
• Onion Routing/Source Routing compatibility
• Key Management in raiden client/transport
• Key Generation, Synapse Key distribution, Key recovery
• Raiden Light Client compatibility

[ulope]

Using olm depends on #4292

[hackaugusto]

duplicated #3893