Repurpose Registry to be a named registry of agents

[ysbaddaden]

Registry should be re-purposed to become a named registry of agents.

Agents would be able to register themselves with an unique name across the application. Supervisors might be able do it for them (especially useful for Dynamic Supervisors, see #7).

Agents would then be capable to communicate with an Agent by name, without holding an actual reference. Ideally, agents could even be (re)started on demand.

This would overcome the current limit that we must hold a reference to an agent, and that all agents must always be running all the time.

Related: #8 and #7