Add Header+Stream pooling

[benaadams]

Headers were previously reused for every request on a connection; now they are created for each request. This override restores the previous behaviour (though slightly different - as it is cross connection and runs with a lower idling memory set)

Ref aspnet/KestrelHttpServer#608

/cc @DamianEdwards

[DamianEdwards]

@halter73 @mikeharder Can we take a look at this one please. This might be the reason we've seen a slight drop-off in the results on the SmurfLab.

[mikeharder]

@DamianEdwards Sure, I will compare plaintext RPS before and after this change.

[benaadams]

The change to "Scenarios": "Plaintext,Json", is likely an oversight

[mikeharder]

@benaadams The only change i see related to "Scenarios": "Plaintext,Json" is an added comma, which is necessary to make valid JSON. Are you talking about the added comma or something else?

[benaadams]

Heh, yeah was just the comma

[mikeharder]

@DamianEdwards @benaadams I'm not seeing a statistically significant change in RPS from this PR. RPS was 980k for dev, and 988k for this PR, with a STDEV of 15k. Is this PR expected to improve plaintext RPS?

[benaadams]

It should revert a change which will reduce sustained perf (eg 5 mins), also you may have to set the numbers to be as high as the connections count

[mikeharder]

@benaadams Is there a benchmark I can run to see the difference resulting from this change?

[benaadams]

Is default now