journeys: add the posibility to have different fallback duration for each mode

Split the "max_duration_to_pt" for each fallback mode. We keep the
"max_duration_to_pt" in the interface for not breaking the compatibily.

The the max duration for car is now set to 30 min.

The "max_duration_to_pt" field in the protobuf is keeped for compatibily
during the migration to the next version. It will be removed in another
pull request.
shouldn't you also put your "we keep this field for compatibily with kraken 1.2, to be removed after the release of the 1.3" comment ?