Add distance restriction to round trip routing

[rabidllama]

Similar to the alternative routes algorithm, GH fallbacks to A-Star Bi-direction, and so there should be a restriction placed on the API that limits the maximum estimated distance of a round trip route to be a value defined in the app.config.

See issue #651 , though implementation should be simpler as it just needs a quick check in the RouteRequestHandler against the value in the app.config as done with other API parameters. A good default would be similar to previous maximum distance with dynamic weights (100km).