Additional state and check in the implementation of PVBUMP

[Vtec234]

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The specification of PVBUMP (fig. 44) keeps track of the protocol version and parameters but not the current epoch. Furthermore, it does not check whether the epoch has changed when invoked. In contrast, the implementation both has the epoch as extra state and checks whether there has been a change in epoch number. This is an entirely sensible thing to do, so this is just to note the difference.

[nc6]

This is changed in https://github.com/input-output-hk/cardano-ledger/pull/753