OS-197 Series of zpool exports and imports can hang the system

Reviewed by: Sarah Jelinek <sarah.jelinek@nexetna.com>
Reviewed by: Saso Kiselkov <saso.kiselkov@nexenta.com>
Reviewed by: Rob Gittins <rob.gittens@nexenta.com>
Reviewed by: Tony Nguyen <tony.nguyen@nexenta.com>
Reviewed by: Alek Pinchuk <alek.pinchuk@nexenta.com>