Currently we hash the entire ConsensusParams and put it in the header as the ConsensusHash.

As we are still planning changes to the ConsensusParams (#2671, #2565), and since there is little need/use for the ConsensusHash right now (especially until we better flesh out the lite-client story), we should reduce the amount of information from the ConsensusParams that are hashed into the ConsensusHash. This will allow us to make changes the ConsensusParams without breaking existing blockchains.

We could consider removing the ConsensusHash altogether, but that seems excessive right now.

Instead, lets change the hash to only include the Block.MaxBytes and Block.MaxGas params.