rework GetClient to use new ForEachClient, return shared_ptr

this simplifies a lot of functions, and removes a lot of potential
errors when using the function. You now only check whether the return
value is null, and if it's not, you have a valid, reference-counted,
client pointer.

include/Client.h

@@ -129,4 +129,5 @@ private:
     std::chrono::time_point<TimeType> mLastPingTime;
 };
 
-std::optional<std::weak_ptr<TClient>> GetClient(class TServer& Server, int ID);
+// Returns a valid client, or nullptr if no such client exists
+std::shared_ptr<TClient> GetClient(class TServer& Server, int ID);