Workhorse: reuse buffers in proxy

What does this MR do and why?

⚠ This MR needs Workhorse: allow testify/assert (!137826 - merged) merged first.

I was browsing the profiler and noticed that Workhorse may make quite a few allocations for the reverse proxy buffer. It's not a lot of memory in absolute terms, but it's a 27% or memory use when not running heavy operations. It's straightforward to optimize this, so I did.

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