rgw/services: don't allocate RGWSI_Notify unless its used

rgw/services: don't allocate RGWSI_Notify unless its used

avoid allocating, initializing, and shutting down RGWSI_Notify if we
aren't going to start or use it

calling RGWSI_Notify::shutdown() without first calling
RGWSI_Notify::do_start() aborts in Finisher::stop() with:
> ceph_abort_msg("join on thread that was never started")

Signed-off-by: Casey Bodley <cbodley@redhat.com>