Fix server getting completely choked up on even a little of DoS

* If client count is unbearable, immediately delete denied clients
* Re-prioritize the checking order of things about incoming clients
* Remove a huge CPU-wasting exception in ReliablePacketBuffer

src/connection.h

@@ -281,7 +281,7 @@ public:
 	u32 size();
 	RPBSearchResult findPacket(u16 seqnum);
 	RPBSearchResult notFound();
-	u16 getFirstSeqnum();
+	bool getFirstSeqnum(u16 *result);
 	BufferedPacket popFirst();
 	BufferedPacket popSeqnum(u16 seqnum);
 	void insert(BufferedPacket &p);