Illumos 4368, 4369.

4369 implement zfs bookmarks
4368 zfs send filesystems from readonly pools
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Garrett D'Amore <garrett@damore.org>

References:
  https://www.illumos.org/issues/4369
  https://www.illumos.org/issues/4368
  https://github.com/illumos/illumos-gate/commit/78f1710

Ported by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2530
Illumos #4374

4374 dn_free_ranges should use range_tree_t
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Max Grossman <max.grossman@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com
Reviewed by: Garrett D'Amore <garrett@damore.org>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Ported by: Tim Chase <tim@chase2k.com>
Illumos 4752

4752 fan out read zio taskqs
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Ported by: Tim Chase <tim@chase2k.com>
Illumos #4754, #4755.

4754 io issued to near-full luns even after setting noalloc threshold
4755 mg_alloc_failures is no longer needed
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Ported by: Tim Chase <tim@chase2k.com>