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

src/Client.cpp

@@ -140,12 +140,14 @@ std::optional<std::weak_ptr<TClient>> GetClient(TServer& Server, int ID) {
     std::optional<std::weak_ptr<TClient>> MaybeClient { std::nullopt };
     Server.ForEachClient([&](std::weak_ptr<TClient> CPtr) -> bool {
         ReadLock Lock(Server.GetClientMutex());
-        if (!CPtr.expired()) {
+        try {
             auto C = CPtr.lock();
             if (C->GetID() == ID) {
                 MaybeClient = CPtr;
                 return false;
             }
+        } catch (const std::exception&) {
+            // ignore
         }
         return true;
     });