MFC r274962:  Replace home-grown CTL IO allocator with UMA.

Old allocator created significant lock congestion protecting its lists
of preallocated I/Os, while UMA provides much better SMP scalability.
The downside of UMA is lack of reliable preallocation, that could guarantee
successful allocation in non-sleepable environments.  But careful code
review shown, that only CAM target frontend really has that requirement.
Fix that making that frontend preallocate and statically bind CTL I/O for
every ATIO/INOT it preallocates any way.  That allows to avoid allocations
in hot I/O path.  Other frontends either may sleep in allocation context
or can properly handle allocation errors.

On 40-core server with 6 ZVOL-backed LUNs and 7 iSCSI client connections
this change increases peak performance from ~700K to >1M IOPS!  Yay! :)

Sponsored by:	iXsystems, Inc.
MFC r275009: Use ctl_set_success() instead of direct inlining.
MFC r275032: Decouple datamove/done logic from CTL status set.
MFC r275058: Coalesce last data move and command status for read commands.

Make CTL core and block backend set success status before initiating last
data move for read commands.  Make CAM target and iSCSI frontends detect
such condition and send command status together with data.  New I/O flag
allows to skip duplicate status sending on later fe_done() call.

For Fibre Channel this change saves one of three interrupts per read command,
increasing performance from 126K to 160K IOPS.  For iSCSI this change saves
one of three PDUs per read command, increasing performance from 1M to 1.2M
IOPS.

Sponsored by:   iXsystems, Inc.
MFC r275365: Move ctlfe_onoffline() out of lock to let it sleep when needed.

Do some more other polishing while there.
MFC r275403: Removed unused variable and unify some names.
MFC r275404: Reduce code duplication by creating ctl_set_res_ua() helper.
MFC r275405: Convert persis_offset from global variable to softc field.
MFC r275447:
Do not pre-allocate reservation keys memory for every possible initiator.

In configurations with many ports, like iSCSI, each LUN is typically
accessed only by limited subset of ports.  Allocating that memory on
demand allows to reduce CTL memory usage from 5.3MB/LUN to 1.3MB/LUN.
MFC r275455: Remove some unused code.
MFC r275458:
Do not pre-allocate UNIT ATTENTIONs storage for every possible initiator.

Abusing ability of major UAs cover minor ones we may not account UAs for
inactive ports.  Allocate UAs storage for port and start accounting only
after some initiator from that port fetched its first POWER ON OCCURRED.

This reduces per-LUN CTL memory usage from >1MB to less then 100K.
MFC r275459: Unify function names after r275458.
MFC r275461:
Increase CTL ports limit from 128 to 256 and LUNs limit from 256 to 1024.

After recent optimizations this change is no longer blocked by CTL memory
consumption.  Those limits are still not free, but much cheaper now.

Relnotes:	yes
Sponsored by:	iXsystems, Inc.
MFC r275474: Add GET LBA STATUS command support to CTL.

It is implemented for LUNs backed by ZVOLs in "dev" mode and files.
GEOM has no such API, so for LUNs backed by raw devices all LBAs will
be reported as mapped/unknown.

Sponsored by:   iXsystems, Inc.