Refactor walker

roshi-walker was exploding (memory) when it encountered a lot of keys to
repair all at once. This branch solves that problem.

- roshi-walker: fix many bugs in the way we traverse the keyspace
- roshi-walker: break apart and sequentialize key processing
- cluster: Score now takes a slice of KeyMembers in a single call
- farm: repair strategies are now composable, with helper wrappers
- pool: add ID method for a given connection pool
- common: remove KeyScoreMembers (it's only used by farm, now)