Better support for large databases

[CasperWA]

An idea here could be to set up a MongoDB with the values normally stored in the Node extras (plus what extra may be needed to complete a structures entry).

Note that this is only recommended for production/static databases. For large local DBs (that might be changed regularly) this is not a feasible solution, as the resulting resources would not always be representative of what's in the actual AiiDA DB, and updating the MongoDB from the AiiDA DB is quite time consuming for large DBs. It could, in theory, be done in idle hours, but this still creates a time difference, where the entities may not represent what's in the AiiDA DB.

Tagging @giovannipizzi for some input on this idea.