Jormungandr drop db

Drop usage of database in Jormungandr:
We check in every instance if the id is in it or not, then we cache it.

To cache we use: https://pythonhosted.org/Flask-Cache/

Thus we no longer need aggregate_places \o/.

Some improvements has to be made: 
 * we need to paralellize calls to kraken.
 * Maybe we can cache per request the region.
 * It would be great if id request to kraken were computed before others

can some response have no publication date ? is so, is it a problem if we don't delete the cache ?