+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.
no, scheduler has on single thread, and since you use `sameThreadExecutionContext` the actual task will run on that thread. I think you have to pass in an explicit ExecutionContext as parameter to the router.