specs: rewrite definition of Read Interface

[lutter]

[Zerim]

A few minor questions/comments but this is looking good!

I'm a little hesitant to merge a PR that replaces a complete & self-consistent design w/ so many open TODO's, but as long as we get them tracked as issues and linked it's OK.

[lutter]

I've added links to the tracking issues to the text.

I agree that it's unfortunate that we have these open to dos in there, but I also think it accurately reflects what we know right now, and that we should be able to fill these fairly easily once the implementation progresses.