watch for protocol parameter changes

[SebastienGllmt]

Although this is related to #147, does not fully overlap

When starting a dApp, you typically need to initially get the genesis information of all the eras. However, for any long-running app, you may run into the following issues:

1. (high priority) a protocol parameter changes while your app is running
2. (low priority) a new era is introduced while your app is running

Although (2) is usually not actionable (i.e. unless the dev adds support for the new era, there's nothing that can be done), (1) is important and you can take immediate action on it

Note: this issue is NOT to match GovernanceActionProposal on transactions. Rather, I want to know when they actually go into effect (which is when apps actually need to react to them)

Note: again, care has to be made in implementing this because of #135