osd/ReplicatedPG: simplify copy-from temp object handling

osd/ReplicatedPG: simplify copy-from temp object handling

Previously the caller was generating a temp object name and passing it
down in severaly different ways.  Instead, generate one when we realize
that we need it, and store it in *one* place (CopyResults), where
the completions can get at the information.

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