BOLT #7: Make channel_update always have htlc_maximum_msat.

[rustyrussell]

trafficstars

I think everyone sets this. Vastly simplifies parsing.

Indeed, every single of the 149964 channel updates seen by my node has his set.

[cdecker]

ACK https://github.com/lightning/bolts/pull/996/commits/466317a300cded545b5f1b0fb6e2191644469a01

[t-bast]

As noted in https://github.com/lightningdevkit/rust-lightning/issues/1515 and discussed during the dev summit, it would also make sense to verify that every implementation sets this value to max_htlc_value_in_flight_msat instead of using the channel capacity.

[t-bast]

Retracting my ACK, I made a proposal at #999 that would benefit from message flags (but I did make htlc_maximum_msat mandatory, which was the goal of this PR).

[arik-so]

I have just downloaded a fresh batch of 134k channel updates from an LND peer; of those channel updates, 364 don't have an HTLC max set, which is 0.27%.

I have separately downloaded 161k updates from a different peer, and got 399 instances of an unset HTLC max, or 0.25%.

[Roasbeef]

Replaced by https://github.com/lightning/bolts/pull/999