Issue #1766 - Lazy evaluation of ChildrenContainer.Children and ChildrenContainer.Stats

[JeffCyr]

ChildrenContainer.Children was returning a copy of all its child. This PR change the Children and Stats property to return the lazy evaluated LINQ expression instead of a copy.

• This is safe because the underlying collection is immutable
• This is more efficient because:
  • No memory is allocated
  • Enumerating the children from the ImmutableDictionary is not expensive
  • The returned IReadOnlyCollection is not iterated in most case

The performance problem of issue #1766 was caused by this line:

akka.net/src/core/Akka/Actor/ActorCell.FaultHandling.cs

Line 422 in d00159e

_actor.SupervisorStrategyInternal.HandleChildTerminated(this, child, GetChildren());

(Note that the implementation of HandleChildTerminated is not touching the children parameter)

With this PR, GetChildren is lazy evaluated, so calling this method is not expensive anymore.

[Aaronontheweb]

Thanks Jeff! I'll review this before I merge the other one

[JeffCyr]

Problem: InternalChildren.Count will not be exact here, it does not take the where filter into account. (Same goes for Stats property).

[JeffCyr]

Problem: InternalChildren.Count will not be exact here, it does not take the where filter into account. (Same goes for Stats property).

I changed the Count to be lazy evaluated too, worst case the children will be enumerated twice if the Count property is accessed.