HAMMER VFS - Improve saturated write performance (2).
* Change the dirty io buffer lists from TAILQs to Red-Black trees.
* The dirty io buffers are sorted by disk address on a flush-group by
flush-group basis and I/O writes are initiated in sorted order.
This significantly improves write I/O throughput to normal HDs.
Essentially what is happening here is that the sheer number of
unsorted buffers are overwhelming the HDs own caches. Having HAMMER
pre-sort the buffers, of which there can be upwards of 100MBs worth,
allow the HD to write more optimally.