Transport e2e encyption

[err508]

We need transport e2e encryption of at least the RevealSecret Message as of #473. Please provide requirements/feedback/opinions/additions in this issue to move it forward.

After discussion with @ulope we came up with two ways of approaching this:

1. Use Matrix libolm library

Ups:

• We don't need to build it
• We don't need to maintain it

Downs:

• coupling with matrix
• we rely on matrix's security

2. Transport agnostic encryption with TLS

Use a Public room/endpoint registry for matrix/udp to publish an X.509 signed by an address to establish a TLS connection between two nodes.

Ups:

• No coupling with matrix
• TLS is battle tested

Downs:

• We have to build it
• We have to maintain it

[andrevmatos]

Please, with any solution, keep in mind development of other possible clients, so it'd be interesting to at least assess its immediate availability on javascript platforms.

[ulope]

Olm depends on #4292