Bolt7: add channel update flag for scid alias

Since lightning#910, nodes are allowed to use aliases instead of real scids. It is
helpful to be explicit about it and set a flag in `channel_update` when
an alias is used instead of a real scid.

We also make the `htlc_maximum_msat` field mandatory: every node on the
network currently sets it, so we can simplify the spec.

07-routing-gossip.md

@@ -413,7 +413,7 @@ of *relaying* payments, not *sending* payments. When making a payment
     * [`u64`:`htlc_minimum_msat`]
     * [`u32`:`fee_base_msat`]
     * [`u32`:`fee_proportional_millionths`]
-    * [`u64`:`htlc_maximum_msat`] (option_channel_htlc_max)
+    * [`u64`:`htlc_maximum_msat`]
 
 The `channel_flags` bitfield is used to indicate the direction of the channel: it
 identifies the node that this update originated from and signals various options
@@ -425,18 +425,13 @@ individual bits:
 | 0             | `direction` | Direction this update refers to. |
 | 1             | `disable`   | Disable the channel.             |
 
-The `message_flags` bitfield is used to indicate the presence of optional
-fields in the `channel_update` message:
+The `message_flags` bitfield is used to provide additional details about how to
+interpret some of the message fields:
 
 | Bit Position  | Name                      | Field                            |
 | ------------- | ------------------------- | -------------------------------- |
-| 0             | `option_channel_htlc_max` | `htlc_maximum_msat`              |
-
-Note that the `htlc_maximum_msat` field is static in the current
-protocol over the life of the channel: it is *not* designed to be
-indicative of real-time channel capacity in each direction, which
-would be both a massive data leak and uselessly spam the network (it
-takes an average of 30 seconds for gossip to propagate each hop).
+| 0             | `must_be_one`             |                                  |
+| 1             | `scid_alias`              | `short_channel_id`               |
 
 The `node_id` for the signature verification is taken from the corresponding
 `channel_announcement`: `node_id_1` if the least-significant bit of flags is 0
@@ -450,7 +445,11 @@ The origin node:
   channel peer, even though the channel has not yet been announced (i.e. the
   `announce_channel` bit was not set).
     - MUST set the `short_channel_id` to either an `alias` it has
-	  received from the peer, or the real channel `short_channel_id`.
+    received from the peer, or the real channel `short_channel_id`.
+    - if it sets `short_channel_id` to an `alias`:
+      - MUST set `scid_alias` to 1 in `message_flags`
+    - otherwise:
+      - MUST set `scid_alias` to 0 in `message_flags`
     - MUST NOT forward such a `channel_update` to other peers, for privacy
     reasons.
     - Note: such a `channel_update`, one not preceded by a
@@ -464,15 +463,8 @@ The origin node:
     - MUST set the `direction` bit of `channel_flags` to 0.
   - otherwise:
     - MUST set the `direction` bit of `channel_flags` to 1.
-  - if the `htlc_maximum_msat` field is present:
-    - MUST set the `option_channel_htlc_max` bit of `message_flags` to 1.
-    - MUST set `htlc_maximum_msat` to the maximum value it will send through this channel for a single HTLC.
-      - MUST set this to less than or equal to the channel capacity.
-      - MUST set this to less than or equal to `max_htlc_value_in_flight_msat`
-    it received from the peer.
-  - otherwise:
-    - MUST set the `option_channel_htlc_max` bit of `message_flags` to 0.
-  - MUST set bits in `channel_flags` and `message_flags `that are not assigned a meaning to 0.
+  - MUST set `must_be_one` in `message_flags` to 1.
+  - MUST set bits in `channel_flags` and `message_flags` that are not assigned a meaning to 0.
   - MAY create and send a `channel_update` with the `disable` bit set to 1, to
   signal a channel's temporary unavailability (e.g. due to a loss of
   connectivity) OR permanent unavailability (e.g. prior to an on-chain
@@ -523,14 +515,11 @@ The receiving node:
     - otherwise:
       - SHOULD queue the message for rebroadcasting.
       - MAY choose NOT to for messages longer than the minimum expected length.
-  - if the `option_channel_htlc_max` bit of `message_flags` is 0:
-    - MUST consider `htlc_maximum_msat` not to be present.
+  - if `htlc_maximum_msat` is greater than channel capacity:
+    - MAY blacklist this `node_id`
+    - SHOULD ignore this channel during route considerations.
   - otherwise:
-    - if `htlc_maximum_msat` is not present or greater than channel capacity:
-      - MAY blacklist this `node_id`
-      - SHOULD ignore this channel during route considerations.
-    - otherwise:
-      - SHOULD consider the `htlc_maximum_msat` when routing.
+    - SHOULD consider the `htlc_maximum_msat` when routing.
 
 ### Rationale
 
@@ -551,12 +540,6 @@ message can rebroadcast it just by changing the `s` component of signature with
 This should however not result in the blacklist of the `node_id` from where
 the message originated.
 
-The explicit `option_channel_htlc_max` flag to indicate the presence
-of `htlc_maximum_msat` (rather than having `htlc_maximum_msat` implied
-by the message length) allows us to extend the `channel_update`
-with different fields in future.  Since channels are limited to 2^32-1
-millisatoshis in Bitcoin, the `htlc_maximum_msat` has the same restriction.
-
 The recommendation against redundant `channel_update`s minimizes spamming the network,
 however it is sometimes inevitable.  For example, a channel with a
 peer which is unreachable will eventually cause a `channel_update` to
@@ -565,6 +548,11 @@ the channel when the peer reestablishes contact.  Because gossip
 messages are batched and replace previous ones, the result may be a
 single seemingly-redundant update.
 
+The `must_be_one` field field in `message_flags` was previously used to
+indicate the presence of the `htlc_maximum_msat` field. This field must
+now always be present, so `must_be_one` is a constant value, and ignored
+by receivers.
+
 ## Query Messages
 
 Negotiating the `gossip_queries` option via `init` enables a number