System should be able to continue when indivual actor exits

[ddrcode]

Right now it's impossible to just stop an individual actor. ctx.stop() stops the system, not actor, that is highly misleading.
System should be able to allow individual actors to quit.
The only scenario when it can happen now (partially) is on overflow, with OverflowPolicy::Fail - broker closes channel in such case, then ActorController closes the actor. It could be better controlled. Pairs with #12

It would require changing Context semantics:

• ctx.exit() - exit actor
• ctx.quite() - stop supervisor