Fix CallbackStack.Node leak (#1318)

Some context to start: Cats Effects has been having memory leaks in CallbackStack since version 3.4.3.
See for example: typelevel/cats-effect#3935

I've been facing this memory leak in an application using fs2-kafka, and found that I'm not the only one (typelevel/cats-effect#3973).

Using a simple consumer > produce stream application, I monitored the size of the CallbackStack using the following command:

```
while sleep 1; do jcmd <pid> GC.class_histogram | grep 'cats.effect.CallbackStack$Node' ; done
```

I found that swapping the `F.race(shutdown, fetch)` for `fetch` stops the memory leak. This should not be an issue because the Stream is anyway interrupted on `.interruptWhen(F.race(shutdown, stopReqs.get).void.attempt)`, but I'm not 100% convinced of this.

Co-authored-by: Adrien Bestel <adrien.bestel@daimlertruck.com>

Author: abestel

modules/core/src/main/scala/fs2/kafka/KafkaConsumer.scala

@@ -170,7 +170,7 @@ object KafkaConsumer {
             stopReqs <- Deferred[F, Unit]
           } yield Stream
             .eval {
-              def fetchPartition: F[Unit] = F
+              val fetchPartition: F[Unit] = F
                 .deferred[PartitionResult]
                 .flatMap { deferred =>
                   val callback: PartitionResult => F[Unit] =
@@ -202,19 +202,15 @@ object KafkaConsumer {
 
                     assigned.ifM(storeFetch, completeRevoked)
                   } >> deferred.get
-                  F.race(shutdown, fetch)
-                    .flatMap {
-                      case Left(()) =>
-                        stopReqs.complete(()).void
 
-                      case Right((chunk, reason)) =>
-                        val enqueueChunk = chunks.offer(Some(chunk)).unlessA(chunk.isEmpty)
+                  fetch.flatMap { case (chunk, reason) =>
+                    val enqueueChunk = chunks.offer(Some(chunk)).unlessA(chunk.isEmpty)
 
-                        val completeRevoked =
-                          stopReqs.complete(()).void.whenA(reason.topicPartitionRevoked)
+                    val completeRevoked =
+                      stopReqs.complete(()).void.whenA(reason.topicPartitionRevoked)
 
-                        enqueueChunk >> completeRevoked
-                    }
+                    enqueueChunk >> completeRevoked
+                  }
                 }
 
               Stream