Drop the required `channel_update` in failure onions

[TheBlueMatt]

As noted previously, channel_updates in the onion failure packets are massive gaping fingerprintign vulnerabilities - if a node applies them in a publicly-visible way the err'ing node can easily identify the sender of an HTLC.

While the updates are still arguably marginally useful for nodes to use in their pathfinding local to retires of the same payment, this too will eventually become an issue with PTLCs. Further, we shouldn't be letting nodes get away with delaying payments by failing to announce the latest channel parameters or enforcing new parameters too soon, so treating the node as having indicated insufficient liquidity (or other general failure) is appropriate in the general case.

Thus, here, we begin phasing out the channel_update field, requiring nodes ignore it entirely and making it optional (though obviously nodes should still provide it for some time).