OpenZFS 8414 - Implemented zpool scrub pause/resume

Authored by: Alek Pinchuk <apinchuk@datto.com>
Reviewed by: George Melikov <mail@gmelikov.ru>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Approved by: Dan McDonald <danmcd@joyent.com>
Ported-by: Alek Pinchuk <apinchuk@datto.com>

OpenZFS-issue: https://www.illumos.org/issues/8414
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/c29616076
Closes #6538