We have to map many index item keys down to a lock that then has a start
and end key range. We also use this mapping over in index item locking
to avoid trying to acquire locks multiple times.
We were duplicating the mapping calculation in these two places. This
refactors these functions to use one range calculation function. It's
going to be used in future patches to fix the mapping of the size index
items.
This should result in no functional changes.
Signed-off-by: Zach Brown <zab@versity.com>