8115 parallel zfs mount

Mounting of filesystems in "filesystem/local" is done using zfs mount -a,
which mounts each filesystems serially. The bottleneck for each mount is
the I/O done to load metadata for each filesystem. As such, mounting
filesystems using a parallel algorithm should be a big win, and bring down
the runtime of "filesystem/local"'s start method.

illumos/illumos-gate@591e0e133f9980083db5d64ac33a30bcc3382ff7

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
Approved by: Matt Ahrens <mahrens@delphix.com>
Author: Sebastien Roy <seb@delphix.com>