discovery: create deDupedAnnouncements struct in gosspier.go

For Part 1 of Issue #275. Create isolated private struct in
networkHandler goroutine that will de-duplicate
announcements added to the batch. The struct contains maps
for each of channel announcements, channel updates, and
node announcements to keep track of unique announcements.

The struct has a Reset method to reset stored announcements, an
AddMsg(lnwire.Message) method to add a new message to the current
batch, and a Batch method to return the set of de-duplicated
announcements.

Also fix a few minor typos.

lnwire/short_channel_id.go

@@ -1,6 +1,6 @@
 package lnwire
 
-// ShortChannelID represent the set of data which is needed to retrieve all
+// ShortChannelID represents the set of data which is needed to retrieve all
 // necessary data to validate the channel existence.
 type ShortChannelID struct {
 	// BlockHeight is the height of the block where funding transaction