Replace reference counting with boxes
Replace reference counting with boxes

The user of the library didn't need any of the information previously
contained in the LcmSubscriber and not having any Rc in there makes it
easier to implement Send and Sync things.

adeschamps/lcm,adeschamps/lcm,adeschamps/lcm,adeschamps/lcm,adeschamps/lcm,adeschamps/lcm,adeschamps/lcm,adeschamps/lcm