- Get rid of _lock() and _unlock(); _lock() is inlined, and so is
  _unlock(), after factoring out some common code at the end of
  tpc_begin() and tpc_end().

- In the refactored _unlock() code, only call self.timeout.end() if
  self.locked was actually set.

In the TimeoutThread class:

- Add some comments.

- Add some assertions.

- Get rid of the stop() method; we're a daemon thread so we'll be
  killed anyway; close_server() is only used from the test suite.

- Switch from using a lock + an event to a condition variable.  Be
  religious about doing stuff only while holding the lock.

- Inline the timeout() function; it shouldn't reacquire the lock
  anyway.

--Guido & Jeremy