Report I/O stats from the CAM_IOSCHED_DYNAMIC extension

Report, on a periodic basis, the I/O latencies the CAM I/O scheduler
computes. These times are only for the hardware portion of the I/O as
measured from the time the operation is scheduled with the SIM using
xpt_action() until the SIM reports it has completed with xpt_dine(). Any
time the I/O operation spends in a software queue is no included.

The P50 (median), P90, P99 and P99.9 statistics about the latency of
each of the read, write and trim operations that completed during the
polling interval are reported. If there are fewer than 2, 10, 100 or
1000 operations during the polling interval, no statistic is reported
and a single dash '-' is displayed.

The read, write and trim commands (either on the command line or at run
time) toggle display of these operations. The color command toggles
color (it defaults to on, like gstat). When color is enabled, unknown
statistics are reported in blue, high latency for a statistics is
reported in red, medium in magenta and low in green (as with gstat). The
med= and hi= commands can set these latency thresholds.

Limitations: The entire sysctl space for all the devices is walked for
each polling period. This should be optimized to remember the OIDs and
only do such polling with the xpt generation changes. There is also no
way to filter devices displayed. This command only works on physical
devies that are connected to SCSI, ATA or NVME sims as those are the
only ones that are instrumented in the CAM I/O scheduler (the
CAM_IOSCHED_DYNAMIC option must be in the kernel, and the dynamic
scheduler can't be disabled).

MFC After: 1 month
Relnotes: yes
Sponsored by: Netflix
Reviewed by: pauamma_gundo.com, chs
Differential Revision: https://reviews.freebsd.org/D34259