TaskThread marks threads as Actors incorrectly

When using `TaskThread`, there are several issues which mean it seems like there are more actors present that there are. 

`Celluloid.stack_dump` shows N tasks + 1 actors in the output, due to thinking that a Thread is an actor only because it has the `:celluloid_actor` Thread-local. 
This also affects `Celluloid::Actor.all`. 

It should be possible to build on #224 and support querying a `Celluloid::Thread` to discover it's reason for being.