Previously, we would list the segments container before completing a
multipart upload so that we could verify ETags and sizes before
attempting to create the SLO. However, container listings are only
eventually-consistent, which meant that clients could receive a 400
response complaining that parts could not be found, even though all
parts were uploaded successfully.
Now, use the new SLO validator callback to validate segment sizes, and
use the existing SLO checks to validate ETags.
Change-Id: I57ae6756bd5f06b80cf03a6b40bf58c845f710fe
Closes-Bug: #1636663