[#456] Build a response proper response for the device notification

* The current JSON library we're using can't handle the serialization
  properly
* When the resp contains data it generates somethin like:
  {"missingUnknown":[],
   "missingFiles":[{"bytes":[{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{}],
   "empty":false}]}
[#456] Modify the delay between attempts

* We now test after 5 or 10 min (2nd and 3rd attempt)
* We log when gave up and save the file as "missing"