Illumos 4101, 4102, 4103, 4105, 4106

4101 metaslab_debug should allow for fine-grained control
4102 space_maps should store more information about themselves
4103 space map object blocksize should be increased
4105 removing a mirrored log device results in a leaked object
4106 asynchronously load metaslab
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Sebastien Roy <seb@delphix.com>
Approved by: Garrett D'Amore <garrett@damore.org>
Ported by: Tim Chase <tim@chase2k.com>

Porting notes:

Converted a handful of kmem_alloc() calls to kmem_zalloc().
Added KM_PUSHPAGE and TQ_PUSHPAGE where necessary.
Destroy the metaslab group taskq in metaslab_group_destroy()

If the taskq isn't destroyed, at least under ZoL, zdb will never exit.
Illumos 4171, 4172

4171 clean up spa_feature_*() interfaces
4172 implement extensible_dataset feature for use by other zpool features
Reviewed by: Max Grossman <max.grossman@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Approved by: Garrett D'Amore <garrett@damore.org>
Ported by: Tim Chase <tim@chase2k.com>
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>