Add websockets

[brantje]

Pull request checklist

• All tests pass. Demo project builds and runs.
• I have resolved any merge conflicts.

This fixes issue #___.

What's in this pull request?

• Add websocket rpc server
• Add websocket events for blockchain and mempool events

[mar-v-in]

Hey @brantje,
Great that you are looking into this.

A few suggestions from my end:

• Please make use of the newer Client API instead of directly calling into consensus objects.
• Events should not be subscribed just because the websocket connection is opened. Instead subscription should happen through a json rpc command itself. The commands could match to the listener commands in Client API.
• Events should make use of JSON-RPC notifications (as described in §4.1 of the JSON-RPC specification)