Change candidates/kickoutList state DB implementation

[koseoyoung]

We've been storing candidate information by height key, so I change to only store current/next epoch's candidate/kickout information. And the archive node will store all of it.

• one issue: how should we rename the funcs (~byepoch) and API not supporting byEpoch

setCandidate -> store with nextKey (mid of epoch)
setKickout -> store with next Key (end of epoch)

shift -> move next data to current data (start of epoch at createPreState())

getCandidates -> read with currentKey (in consensus, start of epoch, with next key)
getKickouts -> read with currentKey (in consensus, start of epoch, with next key)

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[koseoyoung]

latest commit is that

1. poll protocol interface has 4 func- Delegates/ DelegatesOfNextEpoch/ Candidates/ CandidatesOfNextEpoch
2. In side of consensus and blockchain, it compares tip<-> target and calls a corresponding func (call back func)
3. Because now retrieving only tipEpoch delegate and candidate data is available, fixed some dependency with consensus unit test/ API

[CoderZhi]

what if we do a shift in shiftKickoutList?

[CoderZhi]

Unblock it for now, a lot to modify once we have the native staking implementation.