Provide more explicit system for command callbacks

[hawkerm]

Should generalize the notion of commands returning information to the client.

Discussed making these returns asynchronous and putting more smarts in the client to manage messages, serialize delivery to the client code (to avoid threading concurrency issues), and generally manage the client architecture more robustly.

Radar should be reworked to fit this paradigm as a general available command.

[hawkerm]

@wortzmanb What are your thoughts on this? Should Death/Radar/Scan be separate interface methods which get called on the client semi-asynchronously from their main getNextCommand? This could remove the 'null'/state-management issue with radar, but also force implementing more of a 'memory'/coordination across the class.