Fix params.EpochDays to params.NextEpochDays

[dongsam]

Problem Definition

Currently, end-of-epoch uses params.EpochDays directly when allocating farming rewards. However, if params.EpochDays are changed by Parameter ChangeProposal in the middle of the epoch, the already started epoch will perform farming allocation based on the changed params.EpochDays. Therefore, even if Epoch Days is changed in the middle of the epoch, it is necessary to ensure that the changed Epoch Days can be used in the next epoch without affecting the farming reward allocation of the epoch.

Proposal

To this end, CurrentEpochDays(global state) need to be added to check the current Epoch Days, and the existing params.EpochDays is renamed and repurposed to params.NextEpochDays, so if CurrentEpochDays and params.NextEpochDays are different at the start of the new epoch, then update CurrentEpochDays to params.NextEpochDays

Tasks

• Add CurrentEpochDays or EpochEra
• Rename and fix logics for params.EpochDays to params.NextEpochDays
• Update spec and description

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned

[jaybxyz]

Ref: #89