removeTask is it necessary?

[badb]

It's quite obvious that user should be allowed to cancel computation, ie. paid for previously computed subtasks, but don't accept any other providers issue.
What is not clear is that if there should be special broadcast method indicating that requester doesn't want to compute this task anymore and information about it shouldn't been kept and broadcast further. It may be useful for very long-standing task but generates some additional complexity.

Additional note: we should be sure that when requester decides to cancel task it pays for all remaining computation, ie. results for the subtasks that has already started being computed. Additional question: what should happened when we added redundant computation and some subtasks has been computed by some nodes but not enough to verify them.