We need to extend mi_rprev to unpack key block once on load.
Currently we call _mi_get_last_key for each key read, and if keys are packed,
_mi_get_last_key is expensive.
Implementing this task will speed up index scan in reverse order by 6-15 times.
Copyright (c) 2000, 2015, Oracle Corporation and/or its affiliates. All rights reserved.