MFV r336958: 9337 zfs get all is slow due to uncached metadata

MFV r336958: 9337 zfs get all is slow due to uncached metadata

This project's goal is to make read-heavy channel programs and zfs(1m)
administrative commands faster by caching all the metadata that they will
need in the dbuf layer. This will prevent the data from being evicted, so
that any future call to i.e. zfs get all won't have to go to disk (very
much).

illumos/illumos-gate@adb52d9262f45a04318fc6e188fe2b7f59d989a5

Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Thomas Caputi <tcaputi@datto.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author:     Matthew Ahrens <mahrens@delphix.com>