EIP-2181: Increase Clique's DIFF_INTURN

[soc1c]

This document proposes a patch for the Clique proof-of-authority consensus engine that could be used by Ethereum testing and development networks in the future which improves performance and stability.

resolves goerli#13

partially replaces #1955

thanks to @tkstanczak for helping out with this

[karalabe]

At the specified BLOCK_NUMBER, change the block score (difficulty) calculation for blocks signed in turn DIFF_INTURN to 7. The block score for out-of-turn signers remains at 1 since it just needs to be an arbitrary baseline constant.

The original difficulty numbers were not arbitrary at all. They were chosen so that online in-turn signers cannot be censored out, and also that an "offline" in-turn signer cannot pop in after the fact and try to censor out already published blocks. The original design was that anyone online can influence the chain, and anyone offline cannot.

Re the EIP however, I guess by raising the difficulty to 7, you are raising the number of consecutive blocks that need to be all out of turn (and concurrent on 2 chains) for the network to get stuck. So I think the network would need at least 15 online signers + 7 offline to pull off a possibility of getting stuck (even then, you'd need a network partition too for it).

Isn't this EIP however covering the same solution as the one that reduces the number of required signers though? I.e. if I remove the need for a majority of signers to be present, is this EIP still needed?

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