Don't include time to transmit response back to requester when assessing slow handling of requests.

This is to avoid logging a 'slow' request when the source of delay is the viewer in processing a response.
This is not something we can do much about on the server end - it's server-side delay that we're interested in.
To ensure consistency, this commit also had to refactor and simplify inbound non-poll network request handling, though there should be no functional change.
IOSHttpResponse no longer exposes the Send() method, only classes in OpenSim.Framework.Servers.HttpServer should be doing this.
Only the GetTextureHandler was sending its own response.  Now it leaves this to BaseHttpServer, like all other core handlers.

OpenSim/Framework/Servers/HttpServer/OSHttpResponse.cs

@@ -321,13 +321,12 @@ namespace OpenSim.Framework.Servers.HttpServer
         {
             _httpResponse.Body.Flush();
             _httpResponse.Send();
-            
         }
+
         public void FreeContext()
         {
             if (_httpClientContext != null)
                 _httpClientContext.Close();
         }
-        
     }
 }