rework UDP recv loop, add packet statistics tracking

The UDPServerMain was badly designed and had multiple potential bugs and
unexpected & silent failures. The changes simplify the code here, and
tracks those unexpected failures in app-wide statistics. Also added,
while at that, some more tracking of how many packets and bytes are
sent & received per client

include/Common.h

@@ -148,6 +148,14 @@ public:
 
     static std::string SettingToString(const SettingValue& Value);
 
+    // Keeps track of how many packets we dropped on UDP due to fundamentally being malformed
+    static inline std::atomic_size_t MalformedUdpPackets { 0 };
+    // Keeps track of how many packets we dropped on UDP due to 
+    // 1) not having a valid (known) player id
+    // 2) player disconnecting
+    // 3) packet failing to parse
+    static inline std::atomic_size_t InvalidUdpPackets { 0 };
+
 private:
     static void SetShutdown(bool Val);