Fix Gorouter's debugserver endpoint to allow log level changes at runtime

[MarcPaquette]

We have a debugserver in gorouter + explicit code trying to support reconfiguring the log level on the fly. Being able to do this helps operators + support troubleshoot live issues without having to restart Gorouters. This has been broken since ~2017.

This change allows CF Administrators to change gorouter's logging on the fly to triage issue in much finer detail.

This is related to the change in the debug server https://github.com/cloudfoundry/debugserver/pull/72