Streetnetwork: overflow on time_duration

fix http://jira.canaltp.fr/browse/NAVITIAII-1334

there was bad data leading to huge edge in geopal.

reduce time_duration precision and most of all add a check on overflow not to import huge edge.

We could have split the too-big edge but since this part is meant to be refactored in the short term it is not worth it
You may do e.duration.total_seconds() < std::floor(len / ng::default_speed[nt::Mode_e::Walking])