Improve cancel() method of CZConcurrentOperation.

[geekaurora]

Sometimes operations won't be cancelled because they already executed given
operation executes more than one at a time after cancelling - usually five operations
execute at the same time.

TODO

• Operation executes more than one at a time after cancelling.
• Operation execution order after cancelling isn't as enqueued.

[geekaurora]

Solution

• Use ConcurrentBlockOperation instead.