Adjustments to balancing and iostat

Adjustments to pending after int -> uint64_t change
Re-evaluate vdevs periodically
Add response time to zpool iostat
Remove debug printk's
Only record stats for read io's
Adjust iostat output
Seed the average on first read
Aggregate statistics to root vdevs
Decrease probability of re-evaluation
Extend request stats all io types and iostat
Adjust root vdev stats calc for all io types
Fix some errors in root vdev stats calc
Enable pending stats for all io types
Clear request stats
Added some debugging
Adjusted stats gathering