Illumos 4370, 4371

4370 avoid transmitting holes during zfs send
4371 DMU code clean up
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>
Approved by: Garrett D'Amore <garrett@damore.org>
Ported by: Tim Chase <tim@chase2k.com>
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>
Ported by: Tim Chase <tim@chase2k.com>

Porting notes:

Added spa_feat_refcount_cache_lock to struct spa to protext
spa_feat_refcount_cache since zfsonlinux doesn't have an atomic_swap_64
to update it.