Align CdxRecord attribute names with names the server uses

[Mr0grog]

Our CdxRecord objects have names that do not exactly align with the corresponding field names on the CDX server, or what you receive as column headers if asking for CDX search results in JSON format. We should allow access by those names in addition to the names we currently use (and maybe deprecate the current ones?).

• key should be urlkey
• mime_type should be mimetype
• status_code should be statuscode
• url should be original