Fix folly/io/async/test:async_test - AsyncSocketTest.ConnectionExpiry
Fix folly/io/async/test:async_test - AsyncSocketTest.ConnectionExpiry

Summary: The worker thread just needed to be killed before the `AcceptCallback`. `AsyncServerSocket` calls a method on this callback class in its dtor, so it must either be alive or removed using `removeAcceptCallback`.

Differential Revision: D24972293

fbshipit-source-id: 2c7b6d64c2b9b8c00d03107002fa31e565df2a01

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly