blockchain: fix the write known block check (#404)

Before commit bf2f0d1 ("blockchain: fix inconsistent reorg behavior"), the
write known block only checks the if the external total difficulty (TD) is
higher than local one without following the fast finality rule, so it can reorg
an justified block. That commit replaces the check with reorgNeeded function to
enforce the fast finality rule. However, the reorgNeeded function does not
strictly enforce that the external TD is higher than local one, it allows that
the external TD may be equal to the local one. This leads to warning with an
impossible reorg case.

This commit adds back the TD check combining with the reorgNeeded function to
correcly enforce the rule here.

core/blockchain.go

@@ -1675,7 +1675,7 @@ func (bc *BlockChain) insertChain(chain types.Blocks, verifySeals bool) (int, er
 		)
 		for block != nil && bc.skipBlock(err, it) {
 			externTd = new(big.Int).Add(externTd, block.Difficulty())
-			if bc.reorgNeeded(current, localTd, block, externTd) {
+			if localTd.Cmp(externTd) < 0 && bc.reorgNeeded(current, localTd, block, externTd) {
 				break
 			}
 			log.Debug("Ignoring already known block", "number", block.Number(), "hash", block.Hash())