Clean up EPOCH

[WhatisRT]

EPOCH is very complicated, especially considering that we use a lot of open and record stuff. Some of the logic that is responsible for splitting up rewards into what we pay out and what goes into the treasury is shared with POOLREAP, so it might make sense to factor that out in some way.

[WhatisRT]

I decided against adding a new STS for now, since it's close to the HF and who knows how much work that'll be. But I opened #455 for it.