Fix on epoll-ng when testing in RpcTest shutdown

[Coldwings]

RpcTest.shutdown case with epoll-ng io engine has chance failing.

Checked all code, found two problems:

1. EpollNG is possible still got event just after removed watching, makes wake-up notify context not available when waiting thread canceled. Fix by force notify after canceling.
2. The unit test required background thread have to be execute first, still, during calling sk->shutdown, it have chance to yield thread, makes main thread continue and cause object sk double free. Make it only delete once, and wait for background thread finish by thread join.