MFV r284762: 5269 zpool import slow

MFV r284762: 5269 zpool import slow

illumos/illumos-gate@12380e1e701fda28c9e9f32d01cafb54af279eb5

https://www.illumos.org/issues/5269
  When importing a pool (at boot or with zpool import) with many
  filesystem, the process can take minutes. It doesn't matter whether
  the pool has been exported cleanly or uncleanly.  The problem is that
  each dataset has its own log chain. On import, all datasets have to be
  checked if there are logs to replay.  The idea is to speed up this
  process by paralellizing it.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Arne Jansen <jansen@webgods.de>