ReplicatedPG: promote: first draft pass at doing object promotion

ReplicatedPG: promote: first draft pass at doing object promotion

This is not yet at all complete -- among other things, it will
retry forever on any object which doesn't exist in the underlying
pool. But it demonstrates the approach reasonably clearly.

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