Redesign CouchDB security system

@janl:
>
> * closed by default
> * more fine-grained permissions
> * more options for delegated authentication
>
> Our security system is slowly grown and not coherently designed. We should start over. I have many ideas and opinions, but they are out of scope for this. I think everybody here agrees that we can do better. This *very likely* will *not* include per-document ACLs as per the often stated issues with that approach in our data model.

@davisp:
>Big +1 on this. The auth stuff in our code base is hard to follow and difficult to hold in my brain. Taking a step back to redesign from the ground up would be super awesome.

The issue names no files, tests, or entry points; begin by mapping CouchDB's existing security and authentication implementation. Review the goals of closed-by-default access, finer-grained permissions, and delegated authentication, while noting that per-document ACLs are out of scope. Done would require an agreed redesign scope and implementation plan.

Written by the indexing model from the issue text.