perf(ext/http): Add a sync phase to http serving (#19321)

Under heavy load, we often have requests queued up that don't need an
async call to retrieve. We can use a fast path sync op to drain this set
of ready requests, and then fall back to the async op once we run out of
work.

This is a .5-1% bump in req/s on an M2 mac. About 90% of the handlers go
through this sync phase (based on a simple instrumentation that is not
included in this PR) and skip the async machinery entirely.

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