Execute task management request directly in ctl_queue() context.

From one side it allows to remove CTL_FLAG_TASK_PENDING flag, handling of
which significantly complicates fine-grained locking.  From the other side
it reduces task management requests latency even below then that flag could.
As downside, it denies task management code to sleep, but that is not needed
any way now.

Discussed with:	ken

(cherry picked from commit 85412f13ad15f27a688b56995d0575b5a1063c43)
Introduce fine-grained CTL locking to improve SMP scalability.

Split global ctl_lock, historically protecting most of CTL context:
 - remaining ctl_lock now protects lists of fronends and backends;
 - per-LUN lun_lock(s) protect LUN-specific information;
 - per-thread queue_lock(s) protect request queues.
This allows to radically reduce congestion on ctl_lock.

Create multiple worker threads, depending on number of CPUs, and assign
each LUN to one of them.  This allows to spread load between multiple CPUs,
still avoiging congestion on queues and LUNs locks.

On 40-core server, exporting 5 LUNs, each backed by gstripe of SATA SSDs,
accessed via 6 iSCSI connections, this change improves peak request rate
from 250K to 680K IOPS.

MFC after:	2 weeks
Sponsored by:	iXsystems, Inc.

(cherry picked from commit d4f8a83dc389f588133be15ddc87f99bb4c0c448)
Return task management requests to queued execution, but differently.

Testing shown that both original queued design with separate task queue,
and recent direct execution design had significant flaw: If abort request
arrives just after the victim, the last one may not be in the ooa_queue
yet, and so invisible for the task management function.

Unlike original queued implementation, use same queue for all SCSI and
TASK requests from the same initiator. That avoids races between them:
task functions are always executed in proper time, relatively to other
requests.

(cherry picked from commit 3d17a2a3d2526935c300e7c4aa7ff75ef27e2d17)
Fix typo in r267873.

(cherry picked from commit 57e388ec908c0b88402d8ba6ae43b38fc2de45f4)
Fix build on stable/9.