Waits for task's completion, with optional timeout. Tries to retract the task and execute it in-place, if failed - blocks until the task is completed by another thread. If timeout is zero, tries to retract the task and returns immedially after that.Wait(FTimespan::Zero())_ still tries to retract and execute the task, useIsCompleted()_ to check for completeness. The version w/o timeout is slightly more efficient.