Fix Mongoid query cache data mutation

[bencrouse]

Because the model shares the same underlying BSON::Document instance as the query cache, when you change data on the model, the query cache also changes. So if you do a find for the same model, you get a new instance with mutated data which doesn't match the database.

This is particularly noticable as it relates to our releases functionality, where documents loaded out of the database are intentionally mutated, but not saved.

[bencrouse]

One possible risk here, is the number of objects created in memory. Any better ideas?

[bencrouse]

closing because this has a pretty dramatic effect on memory: