OpenZFS 6878 - Add scrub completion info to "zpool history"

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Authored by: Nav Ravindranath <nav@delphix.com>
Ported-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/6878
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/1825bc5
Closes #4787