JDK HTTP client perf

[mtdowling]

Improve JDK HTTP client transport performance

On my M1 Mac mini:

• Mixed H2/TLS GET 1 MiB + PUT 1 MiB, c=10
  • original: 32.767 ops/s at conn=1, 30.575 at conn=3
  • current: 49.357 ops/s at conn=1, 43.678 at conn=3
  • change: about +51% at conn=1, about +43% at conn=3
• Tiny RPC, lower is better
  • original: 1460.365 us/op
  • current: 1069.535 us/op
  • change: about 27% lower mean latency

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