osd/ReplicatedPG: mirror SnapSet info when promoting head

osd/ReplicatedPG: mirror SnapSet info when promoting head

When we promote the head for an object, get the list of snaps from the
backend pool and construct an appropriate SnapSet.  Note that this is
always placed on the head in the cache pool, since we will have a
whiteout object in this case.

Also note that the SnapSet's list of snapids will not include any snaps
for which there were no clones.  This is fine, since it is only used for
creating clones, and we've already done that.

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