Moved mFinished/is_finished() from LegacyPolledResponder to Responder.

Slightly more robust, adds one boolean to all responders, 99%
of which don't need that though, and an extra call redirection,
but well... We might need it this way when I add the possibility
to abort a transfer.

indra/newview/llxmlrpcresponder.cpp

@@ -205,7 +205,6 @@ void XMLRPCResponder::completedRaw(U32 status, std::string const& reason, LLChan
 			}
 		}
 	}
-	mFinished = true;
 }
 
 LLXMLRPCValue XMLRPCResponder::responseValue(void) const