Commit Message: This builds on #19831 , adding an attempt to use http filter methods to deploy flow-control, so we can pause generation of admin output until clients are ready for it.

This is not ready yet -- @alyssawilk has explained to me how to get the flow-control to work

• use high-watermark callback to set a bit in the filter to avoid sending more data
• use low-watermark callback to clear the above bit and send another chunk, and then queue up a callback for the next chunk (assuming high-water callback is not called).

This pattern should also be in the http cache filter and also the Router, though the latter is a lot more complex.
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