Ensure that all iSCSI sessions are correctly terminated during shutdown.
* Enhances the changes done by r286226 (D3052).
* Add shutdown post sync event to run after filesystem shutdown
(SHUTDOWN_PRI_FIRST) but before CAM shutdown (SHUTDOWN_PRI_DEFAULT).
* Changes iscsi_maintenance_thread to processes terminate in preference to
reconnect.
Reviewed by: trasz
MFC after: 2 weeks
Sponsored by: Multiplay
Differential Revision: https://reviews.freebsd.org/D4429
Previously ISID was changed every time, that made impossible correct
persistent reservation, because reconnected session was identified as
completely new one.
Reviewed by: trasz
MFC after: 1 week
