Swift by default only returns a maximum of 10k objects per object
listing. This is problematic when pruning (and potentially otherwise) as
we may not list valid manifests resulting in us never marking the blobs
belonging to those manfiests as valid. These blobs would then get
erroneously deleted.
Address this by paging through object listings using swift's marker
parameter.
Change-Id: Ida85076b716a7718a8ca5fe50e4fbb90b3a41cbf
1105d2bada