As of #2184 we limit the size of the block in bytes.

Additionally, we'd like to limit the size of the block in terms of some application controlled quantifier of resources, called "gas".

Each DeliverTx returns an amount GasUsed. The total GasUsed by the block must be less than the BlockSize.MaxGas consensus param.

Each CheckTx returns an amount GasWanted. The Application must enforce GasUsed < GasWanted. We must track the GasWanted in the mempool so we can reap txs within the gas limit.

Reap should be limited by Gas & Bytes - whichever limit is reached first.