lnwire+discovery: rename ShortChannelIDEncoding to QueryEncoding

Since the the encoding can be used for multiple different fields, we
rename it here to be more generic.

lnwire/encoding.go

@@ -0,0 +1,17 @@
+package lnwire
+
+// QueryEncoding is an enum-like type that represents exactly how a set data is
+// encoded on the wire.
+type QueryEncoding uint8
+
+const (
+	// EncodingSortedPlain signals that the set of data is encoded using the
+	// regular encoding, in a sorted order.
+	EncodingSortedPlain QueryEncoding = 0
+
+	// EncodingSortedZlib signals that the set of data is encoded by first
+	// sorting the set of channel ID's, as then compressing them using zlib.
+	//
+	// NOTE: this should no longer be used or accepted.
+	EncodingSortedZlib QueryEncoding = 1
+)