parallel zfs mount

[Jack Halford]

Hello all,

Gandi has a need for parallel mounting of zfs datasets, apparently this
has been implemented on illumos [1] and then was ported to ZOL [2]. I'm
wondering if anybody has started any work in this direction for FreeBSD
and maybe just hasn't upstreamed or doesn't have the time for the
review, I'd be open to do that work.

Otherwise I'll probably start porting this patch for FreeBSD myself.

[1] https://www.illumos.org/issues/8115
[2] https://github.com/zfsonlinux/zfs/pull/8092

--jack