Headers remove infrequently used member variables

Motivation:
There are two member variables (addAllVisitor, setAllVisitor) which are likely not to be used in the majority of use cases.

Modifications:
Remove these member variables and rely on a method to return a new object when needed.

Result:
Two less member variables for each DefaultHeaders instance.

codec/src/main/java/io/netty/handler/codec/DefaultHeaders.java

@@ -90,8 +90,6 @@ public class DefaultHeaders<T> implements Headers<T> {
     private final ValueConverter<T> valueConverter;
     private final NameConverter<T> nameConverter;
     private final int bucketSize;
-    private EntryVisitor<T> setAllVisitor;
-    private EntryVisitor<T> addAllVisitor;
     int size;
 
     @SuppressWarnings("unchecked")
@@ -1307,12 +1305,8 @@ public class DefaultHeaders<T> implements Headers<T> {
         return removed;
     }
 
-    /**
-     * Lazy initialization of the visitor which will set all headers.
-     */
     private EntryVisitor<T> setAllVisitor() {
-        if (setAllVisitor == null) {
-            setAllVisitor = new EntryVisitor<T>() {
+        return new EntryVisitor<T>() {
             @Override
             public boolean visit(Entry<T, T> entry) {
                 set(entry.getKey(), entry.getValue());
@@ -1320,15 +1314,9 @@ public class DefaultHeaders<T> implements Headers<T> {
             }
         };
     }
-        return setAllVisitor;
-    }
 
-    /**
-     * Lazy initialization of the visitor which will add all headers.
-     */
     private EntryVisitor<T> addAllVisitor() {
-        if (addAllVisitor == null) {
-            addAllVisitor = new EntryVisitor<T>() {
+        return new EntryVisitor<T>() {
             @Override
             public boolean visit(Entry<T, T> entry) {
                 add(entry.getKey(), entry.getValue());
@@ -1336,8 +1324,6 @@ public class DefaultHeaders<T> implements Headers<T> {
             }
         };
     }
-        return addAllVisitor;
-    }
 
     private final class HeaderEntry implements Map.Entry<T, T> {
         final int hash;