Sub-channel settlement requires channel registration on ajdudicator

[matthiasgeihs]

In the optimistic case we can settle a subchannel into its parent channel without having to go on-chain. However, our state machine logic currently does not allow to settle and withdraw a channel from the final state (no valid transition from phase final to phase withdrawing). We should change the state machine behavior so that it allows settling subchannels without channel registration on-chain.