PixelServer should set cache-control on images being returned to github

It's not complicated to add this, and if it results in even slightly better numbers I'm completely fine with it.

[Reference for Adding Cache-Control Headers](https://stackoverflow.com/questions/22443932/cache-control-no-store-must-revalidate-not-sent-to-client-browser-in-iis7-as)

Looking at https://github.com/Azure/azure-sdk-for-net/pull/8559 PR to update pixel tracking links today I investigated a little bit to see how exactly it works.

What if found is that Github is seems to be caching images according to https://help.github.com/en/github/authenticating-to-github/about-anonymized-image-urls and what I see in the browser so I’m not sure how pixel tracking would work.

What’s interesting is that we did collect some data from pixel-server. But the data we collected looks very suspicious.