Keep track of historical quorum values

[Amxx]

Checkpointing quorum is necessary to avoid quorum changes making old, failed because of quorum, proposals suddenly successful.

PR Checklist

• Tests
• Changelog entry

Re-reviewing

[frangio]

Waiting until next week to discuss a more efficient fix.

[frangio]

The issue with this PR is that it increases the cost of voting when the PreventLateQuorum module is used. It seems like this can be fixed by first checking if the latest checkpoint is the one we're looking for (in quorumNumerator(uint blockNumber)) and in that case avoid the binary search. Generally, it is indeed the latest checkpoint that will be needed for voting. The checkpoint history with binary search is still appropriate for the state function.

[Amxx]

I've implemented an optimistic lookup, that skips the binary search if the checkpoint we are looking for is the last one. However that only concerns quorumNumerator(uint256). This function is mostly called through

function quorum(uint256 blockNumber) public view virtual override returns (uint256) {
    return (token.getPastTotalSupply(blockNumber) * quorumNumerator(blockNumber)) / quorumDenominator();
}

The token.getPastTotalSupply(blockNumber) call will not be optimized the same way

[Amxx]

Not, the parts not covered are mechanism used in cases of migration.