Websocket RPC pitfalls

[hobofan]

#12 added the websocket RPC, but there are some caveats with it:

• Only one subscription per connection right now. Not sure how much impact that actually has with most clients, but should be resolved sooner or later.
• Doubt if the way we consume the stream is the proper way to do it:

  rlay-client/src/rpc/mod.rs

  Line 96 in 93b7670

  Ok(Async::NotReady) => thread::sleep(time::Duration::from_millis(100)),

  
  For now it works well enough as it consumes all outstanding entities as fast as it can, but it introduces a delay of up to 100ms for any incoming entities

[hobofan]

• I think the websocket future streams also don't finish correctly when a websocket connection is closed.