Restructure the cluster configuration.

The configuration now consists of persistent members who are added to and
removed from the chain.  This allows nodes' presence/absence to be monitored
independent of whether they are in the chain.  We also sort the nodes by
suspicion when adding and removing, ensuring we chose good nodes.

Fixes #2
replicant-repair

A tool to validate the invariants of the data directory, repairing them where
possible.