Get decoding diagnostic request from decoding thread of can_bus_t

Get decoding diagnostic request from decoding thread of can_bus_t

Decoding divided in 2 subfunctions dedicated to decode either
pure CAN messages or diagnostic (obd2) message. About now,
a diagnostic request has a name then it will be pushed
on the event_queue as a SimpleMessage. Without name full
details of diagnostic response will be pushed as diagnostic
response. This behavior follows the one from OpenXC.

Change-Id: I255f3f6487fa9511ea47c74606014995a7b0f720
Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>