mon/Monitor: early return if routed request is not found

mon/Monitor: early return if routed request is not found

* early return if routed request is not found in routed_requests.
  reduce the indent level, for better readability.
* do not look up the request twice. for better performance.
* use unique_ptr<> for holding the request, for better readability

Signed-off-by: Kefu Chai <kchai@redhat.com>