ReplicatedPG: copy: switch out the CopyCallback interface

ReplicatedPG: copy: switch out the CopyCallback interface

The tuple was already unwieldy with 4 members; I didn't want to add
more. Instead, create a new CopyResults struct which contains all the
object info and completion data, and pass the retval and a CopyResults*
in the CopyCallbackResults tuple.

Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>