Feature/acksync response writer (#1267)

This implements a "detached" mechanism to get an ack from the runner
once it actually starts to run a function. In this scenario the response
returned back is just a 202 if we placed the function in a specific
time-frame. If we hit some errors or we fail to place the fn in time we
return back different errors.

api/models/error.go

@@ -156,6 +156,16 @@ var (
 		error: errors.New("Async functions are not supported on this server"),
 	}
 
+	ErrDetachUnsupported = err{
+		code:  http.StatusNotImplemented,
+		error: errors.New("Detach call functions are not supported on this server"),
+	}
+
+	ErrCallHandlerNotFound = err{
+		code:  http.StatusInternalServerError,
+		error: errors.New("Unable to find the call handle"),
+	}
+
 	// TODO consider removal. see rationale at uses, or remove if none.
 	ErrContainerExitedEarly = err{
 		code:  http.StatusBadGateway,