+act #13004 Adding TailChopping router

Initial implementation + test for #13004
Does it look ok, or some generalisation with ScatterGatherFirstCompletedRouter should be made instead? 
I will update doc when final solution will appear.
Taking into account missing last line with akka.pattern.after that needs future, does it still apply?
If yes, I don't know how can I avoid it and do it in clean way :(
Any suggestions?