Do not remove newly uploaded objects when cleaning up cache on server.

Issue was found while investigating for #609. When the disk gets
filled up, we would prune non-referenced objects. Since this happens
during an upload of object before a reference is added for uploaded
objects, then we would have an incomplete upload. Further pulls would
result in a missing object error.

The solution is to let 6 hours pass before we can prune an object.  To
implement this fix without this minimum age "trick", we would need to
change protocol and deal with concurrent pushes.