- Server types no longer need to store their own contexts; they are
  stored on the caddy.Instance, which means each context will be
  properly GC'ed when the instance is stopped. Server types should use
  type assertions to convert from caddy.Context to their concrete
  context type when they need to use it.
- Pass the entire context into httpserver.GetConfig instead of only the
  Key field.
- caddy.NewTestController now requires a server type string so it can
  create a controller with the proper concrete context associated with
  that server type.

Tests still need more attention so that we can test the proper creation
of startup functions, etc.