MFC r280004: Give block I/O interface multiple (8) execution threads.

MFC r280004: Give block I/O interface multiple (8) execution threads.

On parallel random I/O this allows better utilize wide storage pools.
To not confuse prefetcher on linear I/O, consecutive requests are executed
sequentially, following the same logic as was earlier implemented in CTL.

Benchmarks of virtual AHCI disk, backed by ZVOL on RAID10 pool of 4 HDDs,
show ~3.5 times random read performance improvements, while no degradation
on linear I/O.

git-svn-id: svn://svn.freebsd.org/base/stable/10@280742 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f