use unique_ptr to hold osrm engine instance
use unique_ptr to hold osrm engine instance

- makes intent more clear that there is a single instance of the engine that is queried.

KnockSoftware/node-osrm,oxidase/node-osrm,Project-OSRM/node-osrm,KnockSoftware/node-osrm,KnockSoftware/node-osrm,Project-OSRM/node-osrm,oxidase/node-osrm,Project-OSRM/node-osrm,oxidase/node-osrm,KnockSoftware/node-osrm