core, network: request blocks instead of headers

Closes #1192

1. We now have CMDGetBlockByIndex, so there's no need to request headers
   first when we can just ask for blocks.
2. We don't ask for headers (i.e. we don't send CMDGetHeaders),
   consequently, we shouldn't react on CMDHeaders.
3. But we still keep on reacting on CMDGetHeaders command as
   there could be a node which needs headers.

pkg/network/server_test.go

@@ -142,15 +142,3 @@ func TestServerNotSendsVerack(t *testing.T) {
 	assert.NotNil(t, err)
 	require.Equal(t, errAlreadyConnected, err)
 }
-
-func TestRequestHeaders(t *testing.T) {
-	var (
-		s = newTestServer(t, ServerConfig{})
-		p = newLocalPeer(t, s)
-	)
-	p.messageHandler = func(t *testing.T, msg *Message) {
-		assert.IsType(t, &payload.GetBlockByIndex{}, msg.Payload)
-		assert.Equal(t, CMDGetHeaders, msg.Command)
-	}
-	s.requestHeaders(p)
-}