Reuse existing existing allocations for HttpRecord and response
HeaderMap where possible.
At request end used allocations are returned to the pool and the pool
and the pool sized to 1/8th the current number of inflight requests.
For http1 hyper will reuse the response HeaderMap for the following
request on the connection.
Builds upon https://github.com/denoland/deno/pull/20770
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Co-authored-by: Matt Mastracci <matthew@mastracci.com>
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