Apply patch (requested by reinoud in ticket #569):
Remove dead code and double check if it's allowed to write out
non-sequential there.
--
Sequential writing goes before normal write. Its not likely to ever go
wrong but there could be a race otherwise.
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Add simplistic UBC async flushing as other FS's seem to do. Its a clutch
and needs to be addressed in UBC properly.
Improve write throttling by doing it on page-size basis. The `standard'
solution would cut up files unnessisary during allocation. It also made
assumptions about ucb wich were propably right but still.

(snj)