Add remote source to validate "add" and "removed" keys

[lockefox]

Currently, the test framework is designed to throw an error if any keys change (add or subtract). But our framework should be flexible enough to allow "adding" with warning and only error when a key vanishes.

• add remote (mongoDB atlas free tier) collection for source-key
• add meta tracker for when key was added for record keeping
• group schema-validation tests into one block and make xfail without Travis/creds