Add read replica option to some accessor API calls

First step towards fixing [ORA-629](https://openedx.atlassian.net/browse/ORA-629)
This introduces a "read replica" option for some of the accessors we use in ORA2 data download.
I've updated the test configuration to use an on-disk SQLite database and Django's `TEST_MIRROR` setting.

@ormsbee please review.
So a note for the calling code: replication lag has happened to us from time to time for various reasons, and you'll likely have to deal with `SubmissionNotFoundError`s unless everything upstream is also using the read-replica to pull everything that references the submission. Replication lag has gotten as bad as hours on occasion.