cam/iosched: Add a counter of I/Os that take too long

Add kern.cam.DEV.UNIT.iosched.too_long (to count I/Os taking too long)
and kern.cam.DEV.UNIT.bad_latency (to set this threshold, defaults to
500ms). Each class of I/O (read, write, trim) has its own counters and
thresholds.

Sponsored by: Netflix
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D46033