Improve ZFS N-way mirror read performance + Load/Locality/Rotation

Freebsd Rev. 256956 by @stevenh - Improve ZFS N-way mirror read performance by using load and locality
information.

Reviewed by: gibbs, mav, will
Sponsored by: Multiplay

References:
  http://svnweb.freebsd.org/changeset/base/256956
  https://github.com/zfsonlinux/zfs/pull/1487
  https://github.com/zfsonlinux/zfs/issues/1803
  http://open-zfs.org/wiki/Features#Improve_N-way_mirror_read_performance

Ported by: Andrew Barnes <barnes333@gmail.com>

Closes #1803