Reset router diagnostic statistics

Using maxctrl it is now possible to clear the statistics of services and servers. The command is maxctrl clear statistics target.

$ maxctrl clear statistics MyService

$ maxctrl clear statistics Server1

The system will automatically figure out whether the provided name refers to a server or a service and act accordingly.
If the target is a service, the diagnostics of the associated router will be cleared as well.

Original descrption:
While operating the service through the Maxscale Readwritesplit role,
I want to reset Router Diagnostics information.
It's good to have flush status at runtime while in operation.

1. maxctrl show services
   ...
   Router Diagnostics { ~ ---> flushing }