Avoid object allocations for HTTP2 child streams.

Motivation:

We are allocating a hash map for every HTTP2 Stream to store it's children.
Most streams are leafs in the priority tree and don't have children.

Modification:

 - Only allocate children when we actually use them.
 - Make EmptyIntObjectMap not throw a UnsupportedOperationException on remove, but return null instead (as is stated in it's javadoc).

Result:

Fewer unnecessary allocations.

codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2Connection.java

@@ -38,6 +38,7 @@
 import io.netty.handler.codec.http2.Http2StreamRemovalPolicy.Action;
 import io.netty.util.collection.IntObjectHashMap;
 import io.netty.util.collection.IntObjectMap;
+import io.netty.util.collection.PrimitiveCollections;
 
 import java.util.ArrayList;
 import java.util.Collection;
@@ -216,7 +217,7 @@ private class DefaultStream implements Http2Stream {
         private State state = IDLE;
         private short weight = DEFAULT_PRIORITY_WEIGHT;
         private DefaultStream parent;
-        private IntObjectMap<DefaultStream> children = newChildMap();
+        private IntObjectMap<DefaultStream> children = PrimitiveCollections.emptyIntObjectMap();
         private int totalChildWeights;
         private int prioritizableForTree = 1;
         private boolean resetSent;
@@ -503,6 +504,12 @@ private void notifyHalfClosed(Http2Stream stream) {
             }
         }
 
+        private void initChildrenIfEmpty() {
+            if (children == PrimitiveCollections.<DefaultStream>emptyIntObjectMap()) {
+                children = new IntObjectHashMap<DefaultStream>(4);
+            }
+        }
+
         @Override
         public final boolean remoteSideOpen() {
             return state == HALF_CLOSED_LOCAL || state == OPEN || state == RESERVED_REMOTE;
@@ -539,7 +546,7 @@ final IntObjectMap<DefaultStream> removeAllChildren() {
             totalChildWeights = 0;
             prioritizableForTree = isPrioritizable() ? 1 : 0;
             IntObjectMap<DefaultStream> prevChildren = children;
-            children = newChildMap();
+            children = PrimitiveCollections.emptyIntObjectMap();
             return prevChildren;
         }
 
@@ -561,6 +568,9 @@ final void takeChild(DefaultStream child, boolean exclusive, List<ParentChangedE
                 }
             }
 
+            // Lazily initialize the children to save object allocations.
+            initChildrenIfEmpty();
+
             if (children.put(child.id(), child) == null) {
                 totalChildWeights += child.weight();
                 incrementPrioritizableForTree(child.prioritizableForTree(), oldParent);
@@ -673,10 +683,6 @@ public <V> V remove(Object key) {
         }
     }
 
-    private static IntObjectMap<DefaultStream> newChildMap() {
-        return new IntObjectHashMap<DefaultStream>(4);
-    }
-
     /**
      * Allows a correlation to be made between a stream and its old parent before a parent change occurs
      */

common/src/main/java/io/netty/util/collection/PrimitiveCollections.java

@@ -68,7 +68,7 @@ public void putAll(IntObjectMap<Object> sourceMap) {
 
         @Override
         public Object remove(int key) {
-            throw new UnsupportedOperationException("remove");
+            return null;
         }
 
         @Override