Do not use term supported in channel struct

[lalexgap]

Currently channel struct assumes a supported state is a unanimously signed state. However this isn't strictly true, it depends on the specific ForceMove app rules that define what a supported state is.

For example, with the Virtual Payment App a state with turnNum=2 is supported if it is signed by Bob and contains a signed voucher from Alice.

This doesn't really cause a problem for us because we're only really using the channel struct for setup or finalizing a channel, which does require unanimous consent. However it does mean we're overloading the term supported and making things harder to understand.

To address this I propose that instead of using the term Supported to refer to a fully signed state, we just use FullySigned or UnanimousConsensus.

Perhaps longer term we could consider emulating the EVM and actually calling stateIsSupported but I think that would be overkill at the moment.

[lalexgap]

@geoknee I'd be curious about your thoughts!

[geoknee]

@lalexgap I concur. Let's not use the term where it doesn't currently apply, but let's also think about reinstating it in the future if it becomes important.