Close iSCSI sessions on shutdown

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