fix: Fix leak in FlatMapPrefix operator.

[He-Pin]

extracted from #1621

Motivation:
In #1566 , @queimadus points out a leak

Modification:

1. change the builder from a buffer to a vector builder.
2. free that builder to null after the prefix is generated.

Result:
leak fixed.

import org.apache.pekko.actor.ActorSystem
import org.apache.pekko.stream.scaladsl.{Flow, 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)
    .flatMapPrefix(50000) { _ => Flow[ByteString] }

  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(_))
}

[He-Pin]

@pjfanning Would you like to give this a review?

[pjfanning]

@pjfanning Would you like to give this a review?

Would it be possible to unit test this? We probably don't want anything that takes too much time or memory to run but it might be feasible to add some extra test coverage.

[He-Pin]

I think I can't test it because the builder is protected inside the logic, otherwise, I have to extract the code to a dedicated class and expose it to tests.

Another way is using the MBean, but that is too much.

[He-Pin]

@queimadus Thanks for the reporting, this do spend me a weekend to work out:)