when cancel a coroutine, Mutex should not be poisoned

[Xudong-Huang]

when cancel a coroutine we trigger a cancel panic inside it. This would cause the mutex got poisoned if it not in the wait state. A better and elegant way is to let the mutex finish normally.

this is possible when you hold a mutex guard, and wait for other resources in the coroutine. when the cancel comes, it will run the mutex guard drop in a panicking context while unwind the stack.

and this is only valid in coroutine context, we can't cancel a thread.