Cap gas usage in Before/AfterTx hooks

[larry0x]

@sunnya97 pointed out that under the current design, there is a way to attack the chain: do some very heavy computations in the Before/AfterTx hooks, then throw an error to make the tx fail. Since the tx fails, no gas fee will be paid, but the nodes have been put under heavy loads. A solution to this is to have a max gas cap in the sudo contract calls.