8959 Add notifications when a scrub is paused or resumed

illumos/illumos-gate@301fd1d6f25595cd8c6d6795f39c72d97aff8cd9

Reviewed by: Alek Pinchuk <pinchuk.alek@gmail.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Gordon Ross <gwr@nexenta.com>
Author: Sean Eric Fagan <sef@ixsystems.com>