Cancels this key.
A key that has been canceled is no longer valid. Calling this method on an already canceled key does nothing.
Calling this method is safe at any time. The call might block until another ongoing call to a method of this selector has finished. The reason is that it is synchronizing on the key set of the selector. After this call finishes, the key will have been added to the selectors canceled-keys set and will not be included in any future selects of this selector.