Reduce the Clique SIGNER_LIMIT

[soc1c]

From EIP-1955: ethereum#1955

• The SIGNER_LIMIT was removed from block sealing logic and is only required for voting. This allows the network to continue sealing blocks even if all but one of the validators are offline. The voting governance is not affected and still requires signer majority.

Using this as discussion-to: tracker. See also ref #10

[soc1c]

Removing the SIGNER_LIMIT is dangerous as it would lead straightaway to a list of potential attacks.
Average block time being BLOCK_PERIOD, any signer in a N = 3 signers network can prepare a competing chain. Assume BLOCK_PERIOD is 15 seconds and the signer C decides to stop signing on the same chain as others. Then any 90 seconds that would create A B C A B C in turn blocks would create A B A B A B which gives difficulty of 10. Signer C can prepare C C C C C C chain which would have a difficulty of 10 too. Since C can now arbitrarily choose and accept any random time for signing they can create 7 blocks instead of 6 (by always choosing minimum time) and create an arbitrarily long alternative attack.

ethereum#1955 (comment)