Refer to things in the driver with abstract ids instead of addresses

Refer to things in the driver with abstract ids instead of addresses

Since different protocols can and do have overlapping address spaces, the
address alone isn't sufficient to identify a thing.  Passing both address
and protocol in every function and signal would make the interface too
complicated.

At the moment only locomotives can have a protocol specified, but there's
no technical reason why other types of things couldn't have different
protocols as well.