ZKDelegationTokenSecretManager should handle duplicate Token sequenceNums

The ZKDelegationTokenSecretManager relies on the TokenIdentifier sequenceNumber to identify each Token in the ZK Store. It's possible for multiple TokenIdentifiers to share the same sequenceNumber, as this is an int that can overflow. 

The AbstractDelegationTokenSecretManager uses a Map<TokenIdent, DelegationTokenInformation> so all properties in the TokenIdentifier must match. ZKDelegationTokenSecretManager should follow the same logic.