WIP: time demo poll responses

[mspiegel]

Modify the sensor manager to poll for responses.

Some notes on this branch:

• the signing_service must be started before the signing_proxy. This is because the signing_proxy has not been modified to poll for responses. So the signing_proxy will get stuck waiting on a response to a request that the signing_proxy sent before the signing_service was alive.
• Start the signing_service, then the signing_proxy, and then the sensor_manager. If you kill the signing_service process then the signing_proxy process will crash. I think the request queue on the signing_proxy fills up and the code does not handle that case.

I'm not concerned about these behaviors for an initial demonstration. We can show termination of the signing_proxy and when you restart the signing_proxy then the sensor_manager will continue to function. Alternatively we can implement polling in the signing_proxy. Should be very similar implementation to the polling of the sensor_manager in this branch.