rtr_mgr: add a connection status callback

Add a callback function to the rtr_mgr that allows the library user to
react on connection status changes.

rtr_mgr_init(..) has 2 new parameters:
- A pointer to a function that is called on every connection status
  change.
- A void pointer that is passed to the rtr_mgr_status_fp function. The
  referenced memory area can be used freely by the library user.