This patch implements parallel algotrithm for mounting zfs filesystems. This feature was first implemented on Illumos and ported to ZOL. After asking on the zfs-devel mailing list it was suggested that I start with the ZOL patch.
Mail thread here: https://lists.freebsd.org/pipermail/zfs-devel/2019-February/000516.html
* the ZOL patch pretty much applies directly to FreeBSD (unlike the Illumos patch)
* Had to take some code from the Illumos patch for `#ifdef Illumos` specific code that was removed when ported to ZOL
* After the ZOL port, a fix for a race condition was amended, this is included. (https://github.com/zfsonlinux/zfs/commit/e63ac16d25fbe991a356489c86d4077567dfea21)