kernel - Add TPS balancing mode to scsi_da (da* disks)

kernel - Add TPS balancing mode to scsi_da (da* disks)

* This feature brings write tps down to match read tps when both are
  pending.  Normally both read and write I/Os get at least a few tags,
  but because write I/O is buffered by devices, even giving it one tag
  can complete destroy read performance.

  To accomplish this the feature monitors approximate TPS using a counter
  with exponential decay and will temporarily sets write openings in the
  device queue to zero.

* SSDs usually do a better job on their own, but this feature won't hurt
  so we do not distinguish between HDDs and SSDs.

* New sysctls for this feature (feature is enabled by default):

  kern.cam.da.balance_enable	1	(0=disable, 1=enable)
  kern.cam.da.balance_ratio	100	(read:write ratio, 1:1 is 100)
  kern.cam.da.balance_debug	0	(log to console each second)

  You can adjust the read:write tps ratio as desired.  For example,
  the value 200 will adjust the ratio to 2:1.