[node, contracts] Participants list for defining expected order of parties in an app

[cf19drofxots]

Currently when an app is installed, the list of addresses that are used to determine app state and turn number is sorted.

monorepo/packages/node/src/models/state-channel.ts

Lines 165 to 171 in e730d77

	public getSigningKeysFor(addressIndex: number): string[] {
	return sortAddresses(
	this.userNeuteredExtendedKeys.map(xpub =>
	xkeyKthAddress(xpub, addressIndex)
	)
	);
	}

This is an issue because the state of the app might rely on a different ordering of this list.

For example, it's not clear what address the funds are being sent to here:

monorepo/packages/apps/contracts/ETHUnidirectionalTransferApp.sol

Line 36 in e730d77

return state.transfers[0].to;

If a participants list is used, this list will not be sorted and we can define more reasonable expected transitions.