!act,doc Removing isTerminated from EventStream and ActorClassification

Removing `isTerminated` calls from `EventStream` as well as `ActorClassification`.
This has been reviewed a lot already, and the AC part is guaranteed to be safe I believe.

In the event stream I'm aware of one case where we can end up "watching an actor we don't really need to watch" - which is a specific out of order message delivery to the unsubscriber. Imagine many threads hammering `subscribe(a)` and `unsubscribe(b)` at the same time. We don't guarantee same order of msg delivery as the eventstream updates, so this can happen:

```
// state as seen in eventstream
unsub, sub => sub

// possible message receive ordering:
! unsub => will NOT watch
! sub => will watch
OK

// possible message receive ordering:
! sub => will watch
! unsub => will NOT watch, because it has channels, and the actor checks this explicitly on an volatile field (a costly operation)
OK

// state as seen in eventstream
sub, unsub => unsub

// possible message receive ordering:
! sub => will watch
! unsub => will NOT watch
OK

// possible message receive ordering:
! unsub => will NOT watch
! sub => will watch
NOPE. we're now watching an actor we don't really have to.
```

Such case will not happen in the AC commit, because we enforce linear order of these. The trade of is switching from a mutable datastructure for keeping the subscriptions around, to an immutable pair of `(sequenceId, mappings)`. Not sure if this one edge case in eventstream is worth changing the entire eventstream - as I don't see many cases where such sub/unsub from diff threads but same actor would happen hm...

I have "perf tested" the changes from mutable datastructure to immutable in the `AC`, and details can be found in the pull requests bellow for concrete numbers before / after.

Merged pull requests into one:
* https://github.com/akka/akka/pull/2011
* https://github.com/akka/akka/pull/2037