Consider adopting Handler's asynchronous mode when posting to UI thread

[techyourchance]

RxAndroid exposed an experimental feature that has the potential to improve performance when posting to UI thread.

It's too early to jump to adoption of Handler's async mode in my opinion. However, it might prove to be a good approach.

I'll keep this issue open as a reminder to review RxAndroid's implementation and feedbacks some time in the future, and consider adoption of this mechanism in ThreadPoster.