multi: partition lnwire.ChanUpdateFlag into ChannelFlags and MessageFlags

In this commit:

* we partition lnwire.ChanUpdateFlag into two (ChanUpdateChanFlags and
ChanUpdateMsgFlags), from a uint16 to a pair of uint8's

* we rename the ChannelUpdate.Flags to ChannelFlags and add an
additional MessageFlags field, which will be used to indicate the
presence of the optional field HtlcMaximumMsat within the ChannelUpdate.

* we partition ChannelEdgePolicy.Flags into message and channel flags.
This change corresponds to the partitioning of the ChannelUpdate's Flags
field into MessageFlags and ChannelFlags.

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

lnwire/lnwire_test.go

@@ -608,7 +608,8 @@ func TestLightningWireProtocol(t *testing.T) {
 			req := ChannelUpdate{
 				ShortChannelID:  NewShortChanIDFromInt(uint64(r.Int63())),
 				Timestamp:       uint32(r.Int31()),
-				Flags:           ChanUpdateFlag(r.Int31()),
+				MessageFlags:    ChanUpdateMsgFlags(r.Int31()),
+				ChannelFlags:    ChanUpdateChanFlags(r.Int31()),
 				TimeLockDelta:   uint16(r.Int31()),
 				HtlcMinimumMsat: MilliSatoshi(r.Int63()),
 				BaseFee:         uint32(r.Int31()),