pybind/rados: Fix timeouts for small t

Previously, if passing a number less than 0.5 for
the timeout, operations would always return
a failure status code.

The same problem would also generate premature timeouts
for operations which completed within 0.5s of
their timeout deadline.

Fix the logic so that we only decrement `countdown`
if the thread has not completed when returning from join().

Signed-off-by: John Spray <john.spray@inktank.com>