Allow composing Controllers and Pools

[maxfischer2781]

Being able to use Controllers as part of Pool compositions would allow to defer and reuse control mechanisms. For example, it is straightforward for a pool factory to spawn pools, but controlling them is not. Being able to use Controllers as pool, e.g. to set their (maximum) demand, could simplify this.

FactoryPool -> | Controller -> Pool
               | Controller -> Pool
               | Controller -> Pool
               ...