Design hooks mechanism

We had a very good discussion in Berlin about the hooks architecture. The goal of this ticket is to write a design for the hooks architecture.

A good material to look at is Kea's hooks section in the Developer's Guide. Don't be intimidated with it. It's something that's been in development for years. In Stork, we want to start with something much more modest. But it will grow over time.

The actual implementation is covered by #779 (closed).

EDIT: The design is now available on wiki.