Move all signals search functions into new signals_manager_t object

Move all signals search functions into new signals_manager_t object

Create a class from signals lookup and find standalone function and gather all
find function into it. There is now only 1 function to find either CAN signals
or diagnostic messages, results are returned using an ad-hoc struct containing
vector of one or the other type pointers.

This object also hold subscribed_signals map with events, so this class is a singleton.

Change-Id: I3584c6a91201e6904edc6aeac0abfa1785bdeccc
Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>