Adding comments to cache and generic server

[jackstine]

These are just comments. Please add some comments of your own if you like.

[jackstine]

/kick build-bot here

[nfuden]

nit: Some of these comments are just repeating the exact contents of the functions.
I would argue that method comments should be less implementation linked and talk more about the responsibilities of the method / what it does from a bigger picture.
If you still think that the line by line explantations of functionality (ie using a chan) are needed they can go close to the line itself.

For example with streamenvoyv3 what does it do. It creates a bidirectional stream of envoyv3 configuration which asynchronously retrieves data from the snapshot cache and serves that content.