Rewindable Server

[kazimuth]

Make it so that users can edit what happened earlier in running matches. Possibly in arbitrary matches, if they have the player source code.

One way to do this would be to snapshot the state of user code and the world map in general every set number of frames, and be able to return to snapshots and replay from them. This would depend on #242.

Alternatively, we could just replay the match to that point from the beginning and make the change. This would only be really feasible if the server is super fast, but would require less terrible bytecode changes.

We'd also need to put signals that can encode changes in the schema, and hook things up so the server can understand them, as well as possibly including some way of telling what 'timeline' a particular event from the server happened in, if that makes sense. Like, if you go back and move a unit, we need to resend everything after that.

cc @jbloxham