Once we have the guarantee that a cursor's data will remain valid until the following call to next() we should remove the following:

-The slotsAccessible() and disableSlotAccess() functions in sbe::PlanStage.
-Any copies of data made as part of yield preparation (usually a call to makeOwned()).
-Resetting cursor positions, as part of doSaveState() in leaf nodes (scan.cpp, ix_scan.cpp).
-Tracking of _outerGetNext in the NLJ stage.

This should be do-able in one patch. We will likely be able to remove doSaveState()/doRestoreState() from many of the sbe::PlanStages.