Adds generate_hints=true for dropping hints in response, resolves #1789.

Adds an `generate_hints=false` option which lets us skip generating and
emitting hints for Waypoints. This can be used to decrease the response
size when the user does not need hints anyway.

We should think about making `false` the default here in v6.

include/server/api/base_parameters_grammar.hpp

@@ -141,11 +141,18 @@ struct BaseParametersGrammar : boost::spirit::qi::grammar<Iterator, Signature>
                          add_hint, qi::_r1, qi::_1)] %
                          ';';
 
+        generate_hints_rule =
+            qi::lit("generate_hints=") >
+            qi::bool_[ph::bind(&engine::api::BaseParameters::generate_hints, qi::_r1) = qi::_1];
+
         bearings_rule =
             qi::lit("bearings=") >
             (-(qi::short_ > ',' > qi::short_))[ph::bind(add_bearing, qi::_r1, qi::_1)] % ';';
 
-        base_rule = radiuses_rule(qi::_r1) | hints_rule(qi::_r1) | bearings_rule(qi::_r1);
+        base_rule = radiuses_rule(qi::_r1)   //
+                    | hints_rule(qi::_r1)    //
+                    | bearings_rule(qi::_r1) //
+                    | generate_hints_rule(qi::_r1);
     }
 
   protected:
@@ -157,6 +164,8 @@ struct BaseParametersGrammar : boost::spirit::qi::grammar<Iterator, Signature>
     qi::rule<Iterator, Signature> radiuses_rule;
     qi::rule<Iterator, Signature> hints_rule;
 
+    qi::rule<Iterator, Signature> generate_hints_rule;
+
     qi::rule<Iterator, osrm::engine::Bearing()> bearing_rule;
     qi::rule<Iterator, osrm::util::Coordinate()> location_rule;
     qi::rule<Iterator, std::vector<osrm::util::Coordinate>()> polyline_rule;