Adapt Xenon to use asio changes

Summary: Remove unused m_asyncInvalidCount from Xenon.  Went away with new asio changes.
Added wait sample to Xenon.
Modified Xenon::log to handle wait samples.

Reviewed By: @jano

Differential Revision: D1403928
Band aid for C++ exceptions while unwinding PHP exception.

Reviewed By: @markw65

Differential Revision: D1406669
Fix subtyping "newtype X as Y" with nullables

Summary: The order matters here.

Reviewed By: @elgenie

Differential Revision: D1406629
functorize client stop command

Summary: Functorize Hack client stop command.

Reviewed By: @jwatzman

Differential Revision: D1398492