Support task sleeping

The task suspending mechanism is suspending the task and remove from task queue. User need to revoke manually after suspending.
In some cases, the feature we actually need is let the task sleep a time period and wake up automatically.

To support this, we should re-consider all APIs and the design of task status.

TODOs

Support sleep status and onSleep API
Sleep mechanism
Task introspection APIs refinement

Edited May 18, 2020 by Arren Ping

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