Allow registering LuaActor to actix register

[poga]

Is your feature request related to a problem? Please describe.
Currently, you can't register LuaActor to System or Arbiter since it does not implement SystemService. In order to implement SystemService, LuaActor must impl Default.

The problem is, how do we implement Default for LuaActor? I think the reason actix requires Default for services is that it need to restart them when crashed?

Describe the solution you'd like
Figure out how to implement Default for LuaActor. Then implement SystemService, ArbiterService, and Supervised for it.

Describe alternatives you've considered
Not sure, need to understand more detail on actix's registry.

[poga]

https://github.com/Chris-Ricketts/actix-broker might be a potential solution

[poga]

Also, only one actor per type can be registered to the registry, since LuaActor is a concrete type, we need to work around that.

[poga]

Actix 0.7.10 provides WeakAddr<A> for weak referencing an actor, which can be used to provide custom registry.