Reuse TCP connections for multiple requests, honor Connection close,
and raise wrk health RPS from ~45k to ~80k+ on the same machine.
- Loop HandleConnection up to 1000 reqs/fd with Connection keep-alive
- RawRequest_WantsKeepAlive (HTTP/1.1 default; HTTP/1.0 needs keep-alive)
- Nexus 0.3.0 banner; benches note before/after RPS