Return {ok,CAS::integer()} instead of ok for all store operations.

This saves a roundtrip when you set/add an item that you'll want to modify later.

This is a backward-incompatible change, but it seems important enough and seems to be
a coding oversight as most of the work was already done (but thrown away) in store_callback().

If the compat break (along with a version bump) is not ok or too early, one possibility would be to wrap cberl:set/add/replace to just return 'ok' and let cberl:store return the full info. Another would be to turn the TranscoderOpts into Opts|[Opts] which would give us a flexible API extension mecanism going forward (arguably, if we're breaking compat, doing that strainght away and puting the expiration in the options would be a good thing).