Actor links being maintained on set futures

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When an actor creates a new future for a response it links itself to the resulting future. This creates two problems:

1. As seen in the example on StackOverflow, lots of exit messages can be sent on actor termination, and these messagse may never be cleared out of the receiving actor's queue
2. The links list on the actor expands with every link, and never shrinks back down

Here's the question:
http://stackoverflow.com/questions/2321259/scala-2-8beta1-actor

Fixing #2 should fix #1. I think #2 is really the bigger problem, because it means the links list can expand infinitely, and the because it is maintaining references to the "linked" channel so it can't be GC'd. That could be a big problem in there's some big objects in it.

So I think when the future is set the link needs to be removed.

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Imported From: https://issues.scala-lang.org/browse/SI-3102?orig=1
Reporter: @eengbrec

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@phaller said:
(In r21108) Closes #3102.