Refactor for readability and simplicity
OPLOG schema is needed, and it is fixed.

The OPLOG endpoint needs a schema itself because otherwise we would end
up exposing the AUTH_FIELD when User-Restricted-Resource-Access is
enabled.
Change of mind on OPLOG functionality and configuration.

The design so far:

So now the oplog resource is always used to log deleted documents across
API-maintained collections. In time, this will allow to properly handle
If-Modified-Since requests (#334).

However, oplog will also be capable of logging other methods (POST,
PATCH..) Also, the maintainer will be able to make a 'oplog endpoint'
available to clients. This will be a standard API endpoint (can be
configured with auth, url, etc.). Clients might use it to get
a comprehensive list of updates across all API endpoints since their
last visit.