implements and fixes #997

- ServerPaths object can be populated by passing base path to OSRM
- reduces code/functionality duplication in node-OSRM
remove ServerPaths member from OSRM_impl. pass it by value to c'tor
fix initialization of use_shared_memory
remove shared memory indicator member, use unique_ptr for barriers
rework the population and checking of base paths
Merge pull request #1212 from Project-OSRM/fix_997

implements and fixes #997: support base path for c'tor