This ticket is to enhance testing:

• Make the Oplog class in test_layered23.py available to other tests.
• Enhance Oplog to either take cursors as arguments, or manage cursors, so that we can keep the held open across checkpoint pick-up.
• Write appropriate tests using the enhanced Oplog (further testing SLS-873)
• In new tests, have multiple Btrees, allowing some to have data between checkpoints and some not.

(Note: previously this ticket included work to fix checkpoint pick-up behavior, that work is already covered by SLS-873).