use unique_ptr to hold osrm engine instance

- makes intent more clear that there is a single instance of the engine that is queried.
More unique_ptr
further brushing of the code:

- remove server_path_ptr typedef and member variable
- make class final
- remove emptry dtor
- rename run_query_baton_t
- reorder members in run_query_baton_t
- populate coordinate vector by emplace_back
Merge pull request #75 from Project-OSRM/more_unique

More unique