Orchestra : Replace low-level pyMongo by MongoEngine ODM

[davidfischer-ch]

Using MongoDB through a ODM is the best way to enforce database consistency, simplify our code and make it more robust at the cost of a performance overhead.

• https://mongoengine-odm.readthedocs.org/en/latest/tutorial.html
• http://tobami.net/2011/02/28/benchmarking-mongodb/

This task as is does not add much features to OSCIED, it improve the quality of the code and this is better to make it as soon as possible.

[davidfischer-ch]

Known difficulties

• ObjectId vs UUID string, e.g. _id vs id vs _id : { "$oid" : "(hex)" }
• using Document.to_json() means converting back to dict, then re-converting to json (see orchestra.py ok_200())
• Using Document.to_json() means cannot load_properties
• Does no_dereference=True means foreign keys (id's) not loaded ?

[davidfischer-ch]

Some difficulties solved. Will work on another GitHub issue(s).

[davidfischer-ch]

Or Ming maybe a better idea (Mongo-in-memory) lower-API overhead ... #72