Avoid re-using the WriteBarrier as the Subscription to both the
completionSubscriber and the writeSubscriber.
Instead DownstreamBridge is now called WriteCompletionBarrier and is
both a Subscriber and Subscription, i.e. handles all signals to and
from the completion Subscriber.
This frees the request method implementation in WriteBarrier to assume
it can only be called by the writeSubscriber and hence it's safe to
pass on request signals to the write source outside the synchronized
block.
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