[CELEBORN-1066] Skip looping streamimg sets in numShuffleSteams of Ch…

…unkStreamManager

### What changes were proposed in this pull request?

- Minor improvement in `ChunkStreamManager.numShuffleSteams`, by replacing `.flatMap(Set::stream).count()` to `.mapToLong(Set::size).sum()`

### Why are the changes needed?

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

CI tests.

Closes apache#2013 from bowenliang123/numShuffleSteams.

Authored-by: liangbowen <[email protected]>
Signed-off-by: zky.zhoukeyong <[email protected]>

worker/src/main/java/org/apache/celeborn/service/deploy/worker/storage/ChunkStreamManager.java

@@ -206,6 +206,6 @@ public int numStreamStates() {
 
   @VisibleForTesting
   public long numShuffleSteams() {
-    return shuffleStreamIds.values().stream().flatMap(Set::stream).count();
+    return shuffleStreamIds.values().stream().mapToLong(Set::size).sum();
   }
 }