Use eventlet.sleep inside TriggerWatcher and give other green threads a chance
to perform their work.

If we don't do that, ConsumerMixin will block on waiting a message on a queue,
which means TriggerWatcher will block the whole process and will only work
correctly if it's running in a standalone process (aka won't work with the
sensor wrapper).
Update sensor wrapper to it handles polling and schedueling - move this logic
outside of the sensor class.

Now the sensor class only needs to implement "poll" method.
Also store sensor poll interval in the DB.
Also pass "--poll-interval" argument to the SensorWrapper.
Merge branch 'sensors_process_sandboxing' of github.com:StackStorm/stanley into sensors_process_sandboxing