Allow field effects to wear off

[theastrogoth]

Pending testing, effects are assumed to have a shared duration counter between all raiders, and the duration of an effect is 4x its usual duration in normal play.

Ex: Reflect lasts 20 moves without Light Clay and 32 moves with Light Clay.

Cheers are set to last 3 "turns" (12 moves). In reality, there might be a random element to their duration.

closes #126

[theastrogoth]

This seems to be working, so we can revisit effect durations if/when testing happens