Prevent new speculative attempt after task has issued canCommit to an attempt

When a running attempt calls TaskImpl#canCommit through the taskUmbilical, the TaskImpl will issue a "go" if it is the first attempt to do so. Otherwise it will issue a "no-go". After commitAttempt is assigned is TaskImpl, no other attempt is allowed to succeed at that point. So a speculative attempt that is launched after commitAttempt is assigned can never finished before the original since is will allows be given a "no-go" in the canCommit response. In this jira, I propose to discuss disabling speculative attempts after commitAttempt has been assigned.