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>

routing/pathfind.go

@@ -562,7 +562,7 @@ func findPath(g *graphParams, r *restrictParams,
 		// TODO(halseth): also ignore disable flags for non-local
 		// channels if bandwidth hint is set?
 		isSourceChan := fromVertex == sourceVertex
-		edgeFlags := lnwire.ChanUpdateFlag(edge.Flags)
+		edgeFlags := edge.ChannelFlags
 		isDisabled := edgeFlags&lnwire.ChanUpdateDisabled != 0
 
 		if !isSourceChan && isDisabled {