Get engine state recurringly by requesting a permanent
diagnostic messages request.
Use these information to warn that diagnostic request won't have any responses
at the moment of subscription if engine is off.
Bug-AGL: SPEC-1347
Change-Id: If8bd79bba89acd1c8f5452d3efdbf00a89f8cc77 Signed-off-by: Jonathan Aillet <jonathan.aillet@iot.bzh>