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>

discovery/chan_series.go

@@ -327,7 +327,8 @@ func (c *ChanSeries) FetchChanUpdates(chain chainhash.Hash,
 			ChainHash:       chanInfo.ChainHash,
 			ShortChannelID:  shortChanID,
 			Timestamp:       uint32(e1.LastUpdate.Unix()),
-			Flags:           e1.Flags,
+			MessageFlags:    e1.MessageFlags,
+			ChannelFlags:    e1.ChannelFlags,
 			TimeLockDelta:   e1.TimeLockDelta,
 			HtlcMinimumMsat: e1.MinHTLC,
 			BaseFee:         uint32(e1.FeeBaseMSat),
@@ -346,7 +347,8 @@ func (c *ChanSeries) FetchChanUpdates(chain chainhash.Hash,
 			ChainHash:       chanInfo.ChainHash,
 			ShortChannelID:  shortChanID,
 			Timestamp:       uint32(e2.LastUpdate.Unix()),
-			Flags:           e2.Flags,
+			MessageFlags:    e2.MessageFlags,
+			ChannelFlags:    e2.ChannelFlags,
 			TimeLockDelta:   e2.TimeLockDelta,
 			HtlcMinimumMsat: e2.MinHTLC,
 			BaseFee:         uint32(e2.FeeBaseMSat),