A Connection/Proxy that can be shared between threads

In Async-std, when the executor recognizes that a task is blocking on something, it will automatically spawn a new thread that takes over the asyncronous executor mainloop function.

This means that the current thread may change quietly without intent, which can make using zbus a bit tricky. Therefore, it would be useful for me if Connection and/or Proxy'd objects that own a connection could implement Send in a safe manner.