[governance] Implement handling for DRep expiry

[Sotatek-HuyLe3a]

From the CIP 1694:

"Additionally, registered DReps will need to vote regularly to still be considered active. Specifically, if a DRep does not submit any votes for drepActivity-many epochs, the DRep is considered inactive, where drepActivity is a new protocol parameter. Inactive DReps do not count towards the active voting stake anymore, and can become active again for drepActivity-many epochs by voting on any governance actions or submitting a DRep update certificate..."

DRep expiry rules (WIP):

Each DRep (Delegation Representative) has an expiry epoch that determines how long it remains active. If the current epoch surpasses a DRep’s expiry, the DRep becomes inactive and can no longer participate in governance ratification.

Expiry Calculation

When a DRep is registered or updated, or a DRep submit vote, its expiry is set as:

DRepExpiry = (CurrentEpoch + drepActivity)

CurrentEpoch: The current epoch number.
drepActivity: drepActivity value in protocol parameters

Dormant Epochs: The number of consecutive epochs without governance proposals.

if at an epoch there is no governance action proposal, DormantEpochs increases by 1:

DormantEpochs_new = DormantEpochs_prev + 1

DRepExpiry_new = DRepExpiry_prev + 1

If a new governance proposal appears, DormantEpochs is reset to 0:
DormantEpochs_new = 0

Note:

1. Dormant epoch is not updated for inactive DRep.
2. Dormant epoch is also reset to 0 when DRep is updated