lnwire: add HtlcMaximumMsat field to ChannelUpdate

In this commit, we add a field to the ChannelUpdate
denoting the maximum HTLC we support sending over
this channel, a field which was recently added to the
spec.

This field serves multiple purposes. In the short
term, it enables nodes to signal the largest HTLC
they're willing to carry, allows light clients who
don't verify channel existence to have some guidance
when routing HTLCs, and finally may allow nodes to
preserve a portion of bandwidth at all times.

In the long term, this field can be used by
implementations of AMP to guide payment splitting,
as it becomes apparent to a node the largest possible
HTLC one can route over a particular channel.

This PR was made possible by the merge of #1825,
which enables older nodes to properly retain and
verify signatures on updates that include new fields
(like this new max HTLC field) that they haven't yet
been updated to recognize.

In addition, the new ChannelUpdate fields are added to
the lnwire fuzzing tests.

Co-authored-by: Johan T. Halseth <johanth@gmail.com>

lnwire/lnwire_test.go

@@ -649,13 +649,26 @@ func TestLightningWireProtocol(t *testing.T) {
 		},
 		MsgChannelUpdate: func(v []reflect.Value, r *rand.Rand) {
 			var err error
+
+			msgFlags := ChanUpdateMsgFlags(r.Int31())
+			maxHtlc := MilliSatoshi(r.Int63())
+
+			// We make the max_htlc field zero if it is not flagged
+			// as being part of the ChannelUpdate, to pass
+			// serialization tests, as it will be ignored if the bit
+			// is not set.
+			if msgFlags&ChanUpdateOptionMaxHtlc == 0 {
+				maxHtlc = 0
+			}
+
 			req := ChannelUpdate{
 				ShortChannelID:  NewShortChanIDFromInt(uint64(r.Int63())),
 				Timestamp:       uint32(r.Int31()),
-				MessageFlags:    ChanUpdateMsgFlags(r.Int31()),
+				MessageFlags:    msgFlags,
 				ChannelFlags:    ChanUpdateChanFlags(r.Int31()),
 				TimeLockDelta:   uint16(r.Int31()),
 				HtlcMinimumMsat: MilliSatoshi(r.Int63()),
+				HtlcMaximumMsat: maxHtlc,
 				BaseFee:         uint32(r.Int31()),
 				FeeRate:         uint32(r.Int31()),
 			}