Don't gossip actions which don't change state

[Tom McKenzie]

• When state === nextReducer(state, action) (does not change at all in response to an action), the action is considered "not supported"
• A peer should not replay actions it doesn't support
• This will partially mitigate unsupported actions propagating to the overall network
• Performance implications for scuttlebutts using verification
• Would prevent applications from only storing a subset of the network (such as ignoring others' private messages), so might be optional (opt-out)