Due to the changes in the threading model, the DCB write code can be
simplified by a great amount.
Since only one thread can write to a DCB, it's safe to assume that no new
data is added to the write queue of a DCB while it is being drained. This
removes the need for the code that tracks whether a concurrent DCB write
attempt was made.
Because the high and low water callbacks weren't used by any module, it is
safe to remove them. They offer no real benefits over the drain callback.
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