Real-time referential integrity checking is broken

[eecavanna]

Real-time referential integrity checking was introduced into the validate_json function via PR #835 this week.

The implementation is broken.

In terms of behavior visible to end users:

• It does not take into account data that already exists in the database (❗). As a result, it reports false positives.

In terms of the code:

• The get_collection method that was added to the OverlayDB class only returns the so-called "top" database (it does not return a hybrid top+bottom database).
• An OverlayDB instance is being passed to a function that was designed to receive an instance of pymongo's Database class.

CC: @aclum , @shreddd , @dwinston

[eecavanna]

Given my workload for this week (launching the docs site), I propose this feature be reverted (from the main branch and, as a result, the development environment) until next week.

[aclum]

That is fine with me