`ResponseCookies.Append()` allocates lots, big lots

[dougbu]

Despite using StringValuess, ResponseCookies.Append() is very allocation-heavy. Even just reusing the StringBuilder created in SetCookieHeaderValue.ToString() would be a significant improvement.

For example, running a scenario that adds an Antiforgery token to every response (~156 characters in the token in this case) allocates almost 1 KB extra per response. That's more than 2% of the allocations in this scenario.

Data for 3000 requests:

[muratg]

@dougbu Are you able to send a PR to reuse the StringBuilder?

[dougbu]

Sure. I'll do that today.

[dougbu]

Offline discussion w/ @Tratcher: Reusing a StringBuilder per ResponseCookies instance won't do much because few cookies are set per request. Will instead add an object pool of a small class containing (at least for now) just a StringBuilder instance.

[muratg]

@dougbu That makes sense.