core: split same-td blocks on block height

[karalabe]

Our reorg logic currently is as follows:

• If a new block has a larger TD than our existing head, reorg
• If a new block and our own head have the same TD, flip a coin.

There's a subtle issue in the above algo when it comes to the Clique PoA consensus engine.

In clique, if a node signs a block in-turn, the block's difficulty is 2, whereas if it signs out-of-turn, the resulting difficulty is 1. This means that a chain consisting of 1 in-turn block will weigh the same as a chain consisting of 2 out-of-turn blocks.

This is a rare scenario, but if it occurs, nodes will select the correct chain by flipping a coin, resulting in half the signers on one chain, and half on the other. If there are an odd number of signers, the majority chain will continue. If however the number of signers is even, the two competing chains will get stuck, as neither has the majority to move forward.

This PR adds an extra splitting clause for same-td chains, where the one with the fewer blocks is considered heavier, and only if both are of the same height is the coin flipping done.

[holiman]

LGTM