Enable interrupting parameter setting

by allowing the parametrizable to have _cancel_param method.
closes #330
Make devices abortable

which means bring them to some safe state.
Implement _abort methods in devices
Enable aborting all devices from a concert session
Add abort tests
Implement parameter setting cancellation in motors
Enable gevent futures killing on ctrl-c
Add cancel_operation to futures for cleanup
Add cancel_operation test
Enable interrupting asynchronous parameter setting

with proper cleanup.