UDPServer now uses a single socket which is shared with the
PacketPeerUDP it creates and has a new `poll` function to read incoming
packets on that socket and delivers them to the appropriate peer.
PacketPeerUDP created this way never reads from the socket, but are
allowed to write on it using sendto.
This is needed because Windows (unlike Linux/BSD) does not support
packet routing when multiple sockets are bound on the same address/port.
UDP sockets can be "connected" to filter packets from a specific source.
In case of a bound socket (e.g. server), a new socket can be created on
the same address/port that will receive all packets that are not
filtered by a more specific socket (e.g. the previously connect socket).
This way, a UDPServer can listen to new packets, and return a new
PacketPeerUDP when receiving one, knowing that is a "new client".
