Motivation:
Due some race-condition while handling canellation of TimerTasks it was possibleto corrupt the linked-list structure that is represent by HashedWheelBucket and so produce a NPE.
Modification:
Fix the problem by adding another MpscLinkedQueue which holds the cancellation tasks and process them on each tick. This allows to use no synchronization / locking at all while introduce a latency of max 1 tick before the TimerTask can be GC'ed.
Result:
No more NPE
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