Support changing the gas_limit on the L2 after genesis

During the life of a blockchain, the gas_limit of the L2 could be changed. 

For example, Ethereum had 30M, and increased it to 36M recently. Other l2s may decide to increase it if hardware improves, or have some dynamic block limit if they see spare capacity.

It would be useful to have the ability to alter the gas_limit of a running l2, maybe through a config and a fast restart of the proposer