Auto-remove old gateways and queries

[CasperWA]

In order to remove old gateways and queries, this can be done using a TTL index on last_modified.

More information on a TTL index can be found here.

[CasperWA]

Furthermore, the auto-removal of gateways should rely on usage of said gateway. I don't know if MongoDB has a way of setting up a TTL index to rely on when a document has been retrieved as well or if one has to update the last_modified each time a gateway resource is retrieved from the database? This would have to be updated through the Python code and the motor client.

On the other hand, it might also make sense to cycle gateways after X time no matter what? The main reason here is to keep the databases "fresh", should the base URL change (for whatever reason) or any other information stored.