Make em destroy underlying descriptor objects (and their corresponding libevent

objects and closing the actual fd) asynchronously in the poller thread where
they won't race with libevent internals while polling.

This requires splitting the actual descriptor data from the application owned
handle, because the former needs to have a longer lifetime.

This CL also hacks dualstack_socket_test which is legitimately sensitive to the
delayed close of the listening socket.
	Change on 2014/12/08 by klempner <klempner@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=81603300