Migrate to soketto for Websocket dependency

[Xanewok]

Right now we use (unmaintained) websocket dependency that pulls older versions of hyper (0.10) and tokio (0.1). The soketto dependency is transport-agnostic and so should help reduce the dependency bloat.

Blocked by #603 (migration to futures v0.3).

[Xanewok]

Addressing this will probably also close #158.