Added & modified several message types.

Requests/Callbacks/ICallbackManager.cs

@@ -0,0 +1,9 @@
+namespace HermesSocketLibrary.Requests.Callbacks
+{
+    public interface ICallbackManager<A> where A : class
+    {
+        string GenerateKeyForCallback(A callback);
+        A? Get(string key);
+        A? Take(string key);
+    }
+}