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

This is a port of of @stevenh work, as mentioned in issue: #1803.

I'm still testing this and benchmarking will take me a bit as my test env is virtual and zfs nested, but ztest's etc look to be ok, thought I would PR now so if anyone wants to review changes or assist with benchmarking against master, not ready for any sort of production use as yet.

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