Partial state mirroring

[james gilles]

We gotta send only the visible part of the map from the manager to the client each turn. Also, message arrays should be partially mirrored

Implementation strategies:

• To start, just send the state of all visible units at the beginning of each turn.
• Speedup: send only changed units
  - By cloning the state at the start of the turn and checking for changed units at the end of the turn
  - By having dirty flags for each unit
  The non-map based game state (e.g. research) needs to be replicated in its entirely.

This should ideally be implemented without needing #14 finished first. Make it so that it can be plugged in later.