Remove allocation from ResourceLeakDetector

Motivation:
RLD allocates an ArrayDeque in anticipation of recording access
points.  If the leak detection level is less than ADVANCED though,
the dequeue is never used.  Since SIMPLE is the default level,
there is a minor perf win to not preemptively allocate it.

This showed up in garbage profiling when creation a high number of
buffers.

Modifications:
Only allocate the dequeue if it will be used.

Result:
Less garbage created.

common/src/main/java/io/netty/util/ResourceLeakDetector.java

@@ -27,6 +27,7 @@
 import java.util.Deque;
 import java.util.concurrent.ConcurrentMap;
 
+import static io.netty.util.internal.EmptyArrays.EMPTY_OBJECTS;
 import static io.netty.util.internal.StringUtil.EMPTY_STRING;
 import static io.netty.util.internal.StringUtil.NEWLINE;
 import static io.netty.util.internal.StringUtil.simpleClassName;
@@ -116,7 +117,6 @@ static Level parseLevel(String levelStr) {
         }
     }
 
-    // Should be power of two.
     static final int DEFAULT_SAMPLING_INTERVAL = 128;
 
     /**
@@ -329,7 +329,7 @@ protected void reportInstancesLeak(String resourceType) {
     private final class DefaultResourceLeak extends PhantomReference<Object> implements ResourceLeakTracker<T>,
             ResourceLeak {
         private final String creationRecord;
-        private final Deque<String> lastRecords = new ArrayDeque<String>();
+        private final Deque<String> lastRecords;
         private final int trackedHash;
 
         private int removedRecords;
@@ -347,8 +347,10 @@ private final class DefaultResourceLeak extends PhantomReference<Object> impleme
             Level level = getLevel();
             if (level.ordinal() >= Level.ADVANCED.ordinal()) {
                 creationRecord = newRecord(null, 3);
+                lastRecords = new ArrayDeque<String>();
             } else {
                 creationRecord = null;
+                lastRecords = null;
             }
             allLeaks.put(this, LeakEntry.INSTANCE);
         }
@@ -407,9 +409,14 @@ public String toString() {
 
             final Object[] array;
             final int removedRecords;
-            synchronized (lastRecords) {
-                array = lastRecords.toArray();
-                removedRecords = this.removedRecords;
+            if (lastRecords != null) {
+                synchronized (lastRecords) {
+                    array = lastRecords.toArray();
+                    removedRecords = this.removedRecords;
+                }
+            } else {
+                removedRecords = 0;
+                array = EMPTY_OBJECTS;
             }
 
             StringBuilder buf = new StringBuilder(16384).append(NEWLINE);