Add two offset/lba based AVL trees to the vdev queue
object.

Add a second AVL node within each ZIO so that vdev_queue.c
can sort ZIOs by both type and priority.

Combine reads and writes, irrespecitve of their priorities
into unified, offset sorted, trees.  Selection of the
ZIO to issue is unchanged, but aggregation now uses the
unified tree of the appropriate type so that aggregation
across priority classes is possible.

5313 Allow I/Os to be aggregated across ZIO priority classes