shadir: review metadata validation

urlmd5 is useless: better store url. And if metadata contains url,
file also becomes useless. There remain so little to check that we can
do better with custom validation instead of using a JSON validator.

Also, if we wanted to set a file name on a shadir document, which is
json data, it should be different from the file name of the referenced
cached data. At least, the extension would end with '.json'