Use Boost.Optional instead of custom optional monad implementation.

This switches out the `<variant/optional.hpp>` implementation of the
optional monad to the one from Boost.

The following trick makes sure we keep compile times down:

- use `<boost/optional/optional_fwd.hpp>` to forward declare the
  optional type in header, then include the full blown optional header
  only in the implementation file.

- do the same for the files we touch, e.g. forward declare osmium types,
  allowing us to remove the osmium header dependency from our headers:

      `namespace osmium { class Relation; }

  and then include the appropriate osmium headers in the implementation
  file only. We should do this globally...

References:

- http://www.boost.org/doc/libs/1_59_0/libs/optional/doc/html/index.html
- https://github.com/osmcode/libosmium/issues/123

routing_algorithms/map_matching.hpp

@@ -35,11 +35,14 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include "../util/json_logger.hpp"
 #include "../util/matching_debug_info.hpp"
 
-#include <variant/variant.hpp>
+#include <cstddef>
 
 #include <algorithm>
+#include <deque>
 #include <iomanip>
 #include <numeric>
+#include <utility>
+#include <vector>
 
 namespace osrm
 {