store times in ints instead of longs

[ad510]

I originally stored times as longs because in some previous games I made, it helped prevent overflows when GetTickCount() returned big values. I don't think that will be an issue in Decoherence.

[ad510]

I thought about this some more, and I can't think of benefits of doing this. For example, protobuf-net serializes integers in a way that's independent of whether they're ints or longs. We can reopen this later if we find a reason to do this in the future.