Polish InternalPool as a viable albeit non-default alternative to Spawner

[digitalextremist]

During 0.17.0 the recently displaced InternalPool became Group::Pool and lost its place as the sole and therefore default collection of threads used by ActorSystem and associated things like Future, etc. Basically any thread Celluloid needs.

Let's give it a rest and basically leave it as it is for 0.17.0 ( i.e. less awesome, troublesome, etc ) and revisit it as a viable alternative in 0.17.5.

[digitalextremist]

To actively discourage putting time into Group::Pool right now, it has been removed as a CI test case until we get into this issue later.

[digitalextremist]

Yeah I'm catching intermittent failures, but only on jruby-head and rbx-2 at present. At gym, will look further soon.

[digitalextremist]

@aayushranaut this is a good one to look through, it would give you insight into the actor system itself.

[tarcieri]

Closing as stale. If you are still interested in this issue, please reopen it.