Checksums

Whilst `ValueChecksumError` might not be the best name, I think it makes sense not to use `TLogCheckSumError`. The latter is used to denote on-disk issues: an invalid checksum found in a TLog file, denoting local TLog corruption.

This issue can (with caution, and violating Paxos requirements) be fixed by truncating a TLog.

On the other hand, when a conflict is discovered in the 'rolling' checksum, this doesn't denote an invalid local TLog, rather a local TLog which somehow diverged from the cluster state. Node-local there's nothing really 'wrong', and the case points at a major cluster-wide problem.