osd/ReplicatedPG: include snaps in copy-get results

osd/ReplicatedPG: include snaps in copy-get results

When promoting a snapped object, we need to also get the set of snaps over
which the clone is defined.  This is not strictly available except via the
list-snaps rados call, but that is only used on the snapdir object much
earlier when the head (whiteout) is promoted, and is not conveniently
available now.  Adding it to the internal copy-get is not exposed via
librados (copy-get is not exposed at all) so I don't think this is a
problem.

Signed-off-by: Sage Weil <sage@inktank.com>