Backport JetStream benchmarks improvements to 2.9.x (#4644)

Backporting of JetStream benchmarks that landed in `main` (post 2.10.x)
and `dev` branches back to 2.9.

These improvements are to the benchmark code and not specific to 2.10. 
Therefore it seems appropriate to back port them, so that benchmarks
more accurately compare 2.9 to 2.10 and beyond.

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Signed-off-by: Marco Primi <[email protected]>
Signed-off-by: Reuben Ninan <[email protected]>
Co-authored-by: reubenninan <[email protected]>