discovery: implement ChannelAnnouncement banning

This commit hooks up the banman to the gossiper:
- peers that are banned and don't have a channel with us will get
  disconnected until they are unbanned.
- peers that are banned and have a channel with us won't get
  disconnected, but we will ignore their channel announcements until
  they are no longer banned. Note that this only disables gossip of
  announcements to us and still allows us to open channels to them.

discovery/mock_test.go

@@ -161,3 +161,40 @@ func (s *mockMessageStore) MessagesForPeer(pubKey [33]byte) ([]lnwire.Message, e
 
 	return msgs, nil
 }
+
+type mockScidCloser struct {
+	m           map[lnwire.ShortChannelID]struct{}
+	channelPeer bool
+
+	sync.Mutex
+}
+
+func newMockScidCloser(channelPeer bool) *mockScidCloser {
+	return &mockScidCloser{
+		m:           make(map[lnwire.ShortChannelID]struct{}),
+		channelPeer: channelPeer,
+	}
+}
+
+func (m *mockScidCloser) PutClosedScid(scid lnwire.ShortChannelID) error {
+	m.Lock()
+	m.m[scid] = struct{}{}
+	m.Unlock()
+
+	return nil
+}
+
+func (m *mockScidCloser) IsClosedScid(scid lnwire.ShortChannelID) (bool,
+	error) {
+
+	m.Lock()
+	defer m.Unlock()
+
+	_, ok := m.m[scid]
+
+	return ok, nil
+}
+
+func (m *mockScidCloser) IsChannelPeer(pubkey *btcec.PublicKey) (bool, error) {
+	return m.channelPeer, nil
+}