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.

routing/router.go

@@ -1009,7 +1009,7 @@ type routingMsg struct {
 
 // pruneNodeFromRoutes accepts set of routes, and returns a new set of routes
 // with the target node filtered out.
-func pruneNodeFromRoutes(routes []*Route, skipNode vertex) []*Route {
+func pruneNodeFromRoutes(routes []*Route, skipNode Vertex) []*Route {
 
 	// TODO(roasbeef): pass in slice index?
 
@@ -1130,7 +1130,7 @@ func (r *ChannelRouter) FindRoutes(target *btcec.PublicKey,
 	// aren't able to support the total satoshis flow once fees have been
 	// factored in.
 	validRoutes := make([]*Route, 0, len(shortestPaths))
-	sourceVertex := newVertex(r.selfNode.PubKey)
+	sourceVertex := NewVertex(r.selfNode.PubKey)
 	for _, path := range shortestPaths {
 		// Attempt to make the path into a route. We snip off the first
 		// hop in the path as it contains a "self-hop" that is inserted