lnwire: converge discovery part of messages with specification

Change the name of fields of messages which are belong to the discovery
subsystem in a such way so they were the same with the names that are
defined in the specification.

lnwire/lnwire_test.go

@@ -64,7 +64,7 @@ var (
 	maxUint24 uint32 = (1 << 24) - 1
 	maxUint16 uint16 = (1 << 16) - 1
 
-	someChannelID = ChannelID{
+	someChannelID = ShortChannelID{
 		BlockHeight: maxUint24,
 		TxIndex:     maxUint24,
 		TxPosition:  maxUint16,