+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.
About cycles: I think I followed your first suggestion, but I could have understood something wrong.
Unfortunately I can't see anyway to make index param of function - index is changed in schedule function body.