Replace HashSet with ArrayList as that has two benefits: (#4871)

- ArrayList contains() + add() is faster than HashSet add() for small collections
 - A heap allocation of the iterator is required when iterating HashSet while iterating ArrayList can do with a stack allocation

Signed-off-by: Ludovic Orban <lorban@bitronix.be>

jetty-http2/http2-common/src/main/java/org/eclipse/jetty/http2/HTTP2Flusher.java

@@ -21,6 +21,8 @@ package org.eclipse.jetty.http2;
 import java.io.IOException;
 import java.nio.ByteBuffer;
 import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Collection;
 import java.util.Deque;
 import java.util.HashSet;
 import java.util.Iterator;
@@ -47,7 +49,7 @@ public class HTTP2Flusher extends IteratingCallback implements Dumpable
     private final Queue<WindowEntry> windows = new ArrayDeque<>();
     private final Deque<Entry> entries = new ArrayDeque<>();
     private final Queue<Entry> pendingEntries = new ArrayDeque<>();
-    private final Set<Entry> processedEntries = new HashSet<>();
+    private final Collection<Entry> processedEntries = new ArrayList<>();
     private final HTTP2Session session;
     private final ByteBufferPool.Lease lease;
     private Throwable terminated;
@@ -192,7 +194,10 @@ public class HTTP2Flusher extends IteratingCallback implements Dumpable
 
                         progress = true;
 
-                        processedEntries.add(entry);
+                        // We use ArrayList contains() + add() instead of HashSet add()
+                        // because that is faster for collections of size up to 250 entries.
+                        if (!processedEntries.contains(entry))
+                            processedEntries.add(entry);
 
                         if (entry.getDataBytesRemaining() == 0)
                             pending.remove();