Clean up transaction management for file_complete handler

[BenGalewsky]

Problem

The TransformerFileComplete resource handler is the most critical code in the entire stack. It responds to each file in the dataset being transformed, and is responsible for updating the total number of files processed (either successfully, or failure) - these two counters are how we determine if the transform is complete. The endpoint will be hit repeatedly by all of the running transformers. Consequently, database transaction handing is very important to avoid missing files.

The current implementation uses implicit transactions and doesn't manage locks and flushing to the DB. It's possible that this allows for files to be lost during big transform requests.

Approach

1. Us the DB session to explicitly manage transactions
2. Update the record_file_complete to read the request with with_for_update flag set which will lock the record in the db
3. The increments to files are handled in the same transaction
4. To make the file more readable, the retry call arguments are captured in a single, new decorator.file_complete_ops_retry -

With this decorator, I ran into a problem with unit tests. Importing the module caused the current_app.logger expression to be evaluated. This would throw RuntimeError: Working outside of application context. in the unit tests. Worked around this in the decorator to only access that logger if we are inside the flask app