network: request blocks when there is a ping with bigger than ours height

Turns out, C# node no longer broadcasts an Inv when it's creating a block,
instead it sends a ping and if we're not paying attention to the height
specified there we're technically missing a new block. Of course we'll get it
later after ping timer expiration and regular ping/pong sequence, but that's
delaying it for no good reason.

pkg/network/helper_test.go

@@ -286,6 +286,11 @@ func (p *localPeer) SendPing(m *Message) error {
 	_ = p.EnqueueMessage(m)
 	return nil
 }
+func (p *localPeer) HandlePing(ping *payload.Ping) error {
+	p.lastBlockIndex = ping.LastBlockIndex
+	return nil
+}
+
 func (p *localPeer) HandlePong(pong *payload.Ping) error {
 	p.lastBlockIndex = pong.LastBlockIndex
 	p.pingSent--