Illumos 5351, 5352 - scrub pauses

Illumos 5351, 5352 - scrub pauses

5351 scrub goes for an extra second each txg
5352 scrub should pause when there is some dirty data

Author: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Alex Reece <alex.reece@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Richard Elling <richard.elling@richardelling.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:
  https://www.illumos.org/issues/5351
  https://github.com/illumos/illumos-gate/commit/6f6a76a

Ported-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3383