Slow state channel merges work

[evanmcc]

This issue is meant as a tracker for the various issues and PRs surrounding the speed up and slim down of dispute transactions, and the resolution of the bug that caused so many. Known things so far (edit to add more):

• [ ] speed up dispute merge
• [ ] consider changing dispute close semantics so that we can't have so many duplicative closes
• [ ] make disputed closes smaller, so that there is only a delta instead of two copies of the entire state channel
• [ ] fix cache restart issue
• [ ] make cache use atomics.