rgw: expose the version of synced items to the poster

rgw: expose the version of synced items to the poster

To support this, we add an optional out argument to
RGWMetadatManager::put() and fill in the read_version. When the
function returns, that contains whatever the current on-disk version
of the object is (either what already existed or what we just wrote).

Signed-off-by: Greg Farnum <greg@inktank.com>