Reduce rollback points for MOI to 2

We currently hold 5 rollback points. This originates as a tradeoff between commit interval and rollback period to be covered (~30 seconds). With forestdb, commit interval is not more than 5 seconds for reasonable performance, and so we're holding 5 snapshots. For MOI, the commit interval is 10 minutes – so it makes no sense to hold 5 snapshots. We need to reduce it to 1 snapshot for MOI. (That is: 1 full completed snapshot + 1 partial in-progress snapshot). This will reduce disk usage significantly, and I doubt we'd ever want to rollback more than 10 minutes without an index rebuild being acceptable.