update ping and pong packets

[subtly]

Ping: Add destination endpoint. Rename endpoint property to 'destination' (cosmetic update).
Pong: Add destination endpoint to payload.

[halfalicious]

It looks like this was done at some point.

• PingNode destination:

  aleth/libp2p/NodeTable.h

  Lines 347 to 358 in b867f9e

  	struct PingNode: DiscoveryDatagram
  	{
  	using DiscoveryDatagram::DiscoveryDatagram;
  	PingNode(NodeIPEndpoint const& _src, NodeIPEndpoint const& _dest): DiscoveryDatagram(_dest), source(_src), destination(_dest) {}
  	PingNode(bi::udp::endpoint const& _from, NodeID const& _fromid, h256 const& _echo): DiscoveryDatagram(_from, _fromid, _echo) {}
  	static const uint8_t type = 1;
  	uint8_t packetType() const override { return type; }
  	unsigned version = 0;
  	NodeIPEndpoint source;
  	NodeIPEndpoint destination;

• Pong destination:

  aleth/libp2p/NodeTable.h

  Lines 383 to 391 in b867f9e

  	struct Pong: DiscoveryDatagram
  	{
  	Pong(NodeIPEndpoint const& _dest): DiscoveryDatagram((bi::udp::endpoint)_dest), destination(_dest) {}
  	Pong(bi::udp::endpoint const& _from, NodeID const& _fromid, h256 const& _echo): DiscoveryDatagram(_from, _fromid, _echo) {}
  	static const uint8_t type = 2;
  	uint8_t packetType() const override { return type; }
  	NodeIPEndpoint destination;