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.