Previously, if you uploaded a file as an SLO then re-uploaded it
with the same segment size and mtime, the second upload would
go delete the segments it just (re)uploaded. This was due to
us tracking old_slo_manifest_paths and new_slo_manifest_paths
in different formats; one would have a leading slash while the
other would not.
Now, normalize to the stripped-slash version so we stop deleting
segments we just uploaded.
Change-Id: Ibcbed3df4febe81cdf13855656e2daaca8d521b4