RideCache::load()

.. loads the cache/rideDB.json back on startup to avoid
   any refresh of metrics etc

.. uses a lex/bison JSON parser, not because of performance
   since its only run one at startup, but because;

   * QtJSON support is only available in QT5
   * mvjson uses a DOM model that creates a memory overhead
   * the existing bison/lex parser works well and is easy to
     use as a template for this

We now have a good mechanism for metrics and metadata refresh,
cache and recovery so can look at using it in the charts and
the ride navigator next !