Runtime upgrade

[bkchr]

Substrate supports to upgrade the runtime from within the Blockchain. We need to implement some extended functionality to make the upgrade fit into the Polkadot model of upgrading Parachain runtimes, described here.

• validate_block needs to check that :code is only altered when allowed (see the Polkadot issue for the requirements).
• Provide a pallet in Cumulus that makes the process of registering an upgrade/checking if an upgrade is doable easy to use.
• It would probably be nice to augment set_code etc to fail on applying, if the requirements are not allowed (instead of failing in validate_block). Users can for example send a proposal to upgrade the runtime, this should fail to dispatch if it does not follow the correct path.