ReplicatedPG: only increment active_scrub on primary for final push

ReplicatedPG: only increment active_scrub on primary for final push

We only queue the _applied_recovered_object callback on the primary for the
final push.  It is this callback which decrements active_pushes.  It's ok to
not increment active_pushes for the intermediate pushes since these only affact
a temp file.

Signed-off-by: Samuel Just <sam.just@inktank.com>