Refactor direction engines and unify structure

[hlfan]

Untangling FOSSGISOSRMEngine and moving from pushing for loops to .map() for the other engines.

Description

The three routing engines' code now follows a similar structure:

function ...Engine (id, ...) {
  function _processDirections(...) {...}

  return {
    ...,
    getRoute: function (points, callback) {
      data = {...};
      return $.ajax({
        ...,
        success: function // that calls the callback with _processDirections
      });
    }
  };
};

How has this been tested?

Creating a conditional dev tools breakpoint and shoving it all in there.