Init initializes the read version cache.

NewTokenInvalidError creates a TokenInvalidError using the provided message.

TransactConflicting is the same as db.Transact, but won't retry in case of conflicting transactions and will throw NewTokenInvalidError instead it will also execute onCommit callback if transaction commits sucessfully.

AtomicMax is used to track the max value.

ReadVersionCache caches "safe" read versions, which are known to be.

TokenInvalidError happens when the lease token is no longer valid.