vici: Allow the Python event listen() operation to optionally time out

vici: Allow the Python event listen() operation to optionally time out

The architecture of the Python client is completely blocking, which is fine
for many simple scripts. For more complex applications that do other I/O
and listen for vici events, the most feasible way to integrate the client is
to use a dedicated thread.

Unfortunately, Python has no simple support for thread cancellation. And
having that thread in a blocking recv() does not allow to terminate the
thread gracefully with an Event or the like.

As a way out, add a timeout to the listen() call, so the thread can
periodically do other things, like checking for termination Event and
react on it. Returning from listen() on timeout can be suboptimal, though,
as it involves registration/deregistration for events, including the risk
for missing events while not registered. So return a (None, None) tuple
instead on timeout, allowing the caller to periodically do other things
while staying registered for the events and continue in listen().

The timeout applies to the socket recv() for the start of the header, only,
so a message is either read in full or times out, avoiding the risk
of breaking message framing on the stream with partial reads.