osd/scrub: split on_pg_activate from on_new_interval

osd/scrub: split on_pg_activate from on_new_interval

Separate and clarify handling of interval termination, pg activation,
and configuration change.

A primary PG now registers with its OSD for scrubbing only on
activation: on_pg_activate() called from PG::on_activate().

When the interval ends, the scrubber is notified via on_interval_change,
which is responsible for cleaning up any active or replica state
associated with scrub.

Configuration changes are still handled by update_scrub_job().

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>
Signed-off-by: Samuel Just <sjust@redhat.com>