osd: list pool/bucket contents excludes missing objects
listing pool contents only lists objects stored locally, which excludes anything missing on the primary.
we could include missing in the pgls results, but it's hard to do that in a non-racy way without either missing entries or potentially duplicating them.
objecter: set pgls start_epoch field
For each pg, start out with start_epoch = 0 in the first request. For
subsequent requests, set it to the first reply's epoch. This forces the
OSD to ignore our cookie and "restart" if the pg mapping changes and there
is a possibility of incomplete results.
The price we pay is the possibility of duplicate results.