move cache_key to shipment model
move cache_keys from rates to shipment object

this helps with debugging no rates on shipments, added the following
helpers:
- cache_key, to replace the calculator.cache_key call
- cache_keys, fetches all of the cache_keys available for given rates on
  shipment
- remove_cache_carrier_messages, removes the cache for all of the
  cache_keys on shipment
- get_cache_carrier_messages, outputs the contents of the cached files
  of the rates, this is helpful for debugging