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/onion_error_test.go

@@ -20,7 +20,8 @@ var (
 		Signature:      sig,
 		ShortChannelID: NewShortChanIDFromInt(1),
 		Timestamp:      1,
-		Flags:          1,
+		MessageFlags:   0,
+		ChannelFlags:   1,
 	}
 )