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
bump to OSRM 4.4 release
update links in README.md
pick up latest round of changes to fix link order
yet another git sha bump to OSRM 4.4 release
Pull the corresponding osrm-backend revision by release tag:

- Update environment variable to resemble https://github.com/mapnik/mapnik-packaging/pull/184