Some refactorings in Sink

[mariusmuja]

A few refactorings in Sink, made a code a bit more DRY, added onError and onComplete arguments to Sink.create.

[LukaJCB]

Hey @mariusmuja! Great work! Really appreciate it! However, I'm not sure why exactly we'd need onError and onComplete arguments. The streams used for Sinks are usually infinite and don't include errors, no?

[mariusmuja]

@LukaJCB Yes, that's true. However Sinks can also be used for network communication (websockets for example), in which case network errors can occur. Also, the onError and onComplete arguments have default values, so the Socket.create usage remains the same in most cases.