Consider backpressure

[mehaase]

Preface: I don't know if this issue will be resolved by writing code, writing docs, or both. I'm starting this thread just to document some of the issues around backpressure and potential solutions.

The library has no special mechanics or documentation to deal with the important issue of backpressure. This is an area of concern for me because I once spent a long time debugging a backpressure problem in aaugstin's websockets, and at the time I didn't even know what the term meant!

At the library level, we can avoid backpressure issues by making sure not to have any unbounded buffers inside the library. At the user level, we probably need some documentation so that they are also aware of this issue and can avoid it.