[I] Reduce memory usage when large number of GraphStageLogic involves [pekko]

[via GitHub]

He-Pin opened a new issue, #1624:
URL: https://github.com/apache/pekko/issues/1624

    @queimadus's report in https://github.com/apache/pekko/discussions/1566
   
   and with the code below
   ```scala
   import org.apache.pekko.actor.ActorSystem
   import org.apache.pekko.stream.scaladsl.{Sink, Source}
   import org.apache.pekko.util.ByteString
   
   import scala.concurrent.Await
   
   object PekkoQuickstart extends App {
     private implicit val system: ActorSystem = ActorSystem()
   
     val s = Source
       .repeat(())
       .map(_ => ByteString('a' * 400000))
       .take(1000000)
       .prefixAndTail(50000)
       .flatMapConcat { case (prefix, tail) => Source(prefix).concatLazy(tail) }
   
     val r = Source.empty
       .concatAllLazy(List.tabulate(30000)(_ => s): _*)
       .runWith(Sink.ignore)
   
     Await.result(r, scala.concurrent.duration.Duration.Inf)
     println(r.value)
   
   //  Source
   //    .repeat(s)
   //    .take(30000)
   //    .flatMapConcat(x => x)
   //    .runWith(Sink.ignore)
   //    .onComplete(println(_))
   
   //  Source.empty
   //    .concatAllLazy(List.tabulate(30000)(_ => Source.lazySource(() => s)): 
_*)
   //    .runWith(Sink.ignore).onComplete(println(_))
   }
   ```
   we can get a heap dump of 
   
![image](https://github.com/user-attachments/assets/7ca996c8-5168-4efa-a794-ac9ba165301d)
   
   To fix the problem, I think we need to clean the logic once we are done with 
a sub-graph, but the current code needs to get a snapshot for materializer
   


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