Of particular interest is to run upgrade/downgrade tests with both the mmapv1 and wiredTiger storage engines. The downgrade_replset.js test starts a replica set with the "latest" version and then downgrades all of the nodes to the "last-stable" version; however, it only runs with the mmapv1 storage engine. It therefore doesn't provide any coverage of the backwards breaking change from KeyString v2 (SERVER-19703).

Note: The work for this ticket may also involve tagging the existing multiversion tests with the "requires_persistence" tag to skip running them against the ephemeralForTest and inMemory storage engines.