Rework packet receiving in ServerThread

Notably it tries to receive all queued packets
between server steps, not just one.

src/network/networkpacket.h

@@ -35,6 +35,7 @@ public:
 	~NetworkPacket();
 
 	void putRawPacket(u8 *data, u32 datasize, session_t peer_id);
+	void clear();
 
 	// Getters
 	u32 getSize() const { return m_datasize; }