cvmfs_swissknife gc has no memory of what it has deleted

[mharvey-jt]

Debugging the garbage collection of a repo today (with 2.8.1) I observe that gc appears not to retain any memory of objects that it has already deleted, leading to (in our case very many) redundant DELETE calls to object storage, and much slower performance than expected.

[jblomer]

Good point! We keep the hash set of the preserved items but not of the already deleted items. That would be a very nice performance improvement!