bor: re-visit logic of mutex lock/unlock in `finality`

[AskAlexSharov]

Need re-visit this code and remove if possible. Or move this logic to 1 level down - where can lock+defer unlock mutex inside 1 func.

isLocked := service.LockMutex(endBlockNr)

if !isLocked {
	service.UnlockMutex(false, "", endBlockNr, common.Hash{})
	return false, errors.New("whitelisted number or locked sprint number is more than the received end block number")
}

if localEndBlockHash != hash {
	service.UnlockMutex(false, "", endBlockNr, common.Hash{})
	return false, fmt.Errorf("hash mismatch: localChainHash %s, milestoneHash %s", localEndBlockHash, hash)
}

service.UnlockMutex(true, milestoneId, endBlockNr, localEndBlock.Hash())