Illumos 3641

3641 want a histogram of compressed block sizes
Reviewed by: George Wilson <gwilson@zfsmail.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Approved by: Eric Schrock <eric.schrock@delphix.com>

Ported by: Tim Chase <tim@chase2k.com>
WIP 4101-4106
WIP illumos 4101-4106
WIP illumos 4101-4106