Yield and initial state for MapAsync and MapThreaded

[semiversus]

MapAsync and MapThread are stateless and the first value emitted is after running the first coroutine/function. By adding an init argument the operator get's stateful with init as initial state.

Additionally explore the use of yield in the coroutine to emit values during the coroutine/function is running.

[semiversus]

Instead of yield it's easier to use self.notify(). For MapThreaded a threadsafe variant with loop.call_soon should be implemented.