Fix a bug in internalNioBuffer() implementations of derived buffers

- A user can create multiple duplicates of a buffer and access their internal NIO buffers. (e.g. write multiple duplicates to multiple channels assigned to different event loop.)  Because the derived buffers' internalNioBuffer() simply delegates the call to the original buffer, all derived buffers and the original buffer's internalNioBuffer() will return the same buffer, which will lead to a race condition.
- Fixes #1739

buffer/src/main/java/io/netty/buffer/DuplicatedByteBuf.java

@@ -289,7 +289,7 @@ public class DuplicatedByteBuf extends AbstractDerivedByteBuf {
 
     @Override
     public ByteBuffer internalNioBuffer(int index, int length) {
-        return buffer.internalNioBuffer(index, length);
+        return buffer.internalNioBuffer(index, length).duplicate();
     }
 
     @Override

buffer/src/main/java/io/netty/buffer/ReadOnlyByteBuf.java

@@ -292,7 +292,7 @@ public class ReadOnlyByteBuf extends AbstractDerivedByteBuf {
 
     @Override
     public ByteBuffer internalNioBuffer(int index, int length) {
-        return buffer.internalNioBuffer(index, length);
+        return buffer.internalNioBuffer(index, length).duplicate();
     }
 
     @Override

buffer/src/main/java/io/netty/buffer/SlicedByteBuf.java

@@ -271,7 +271,7 @@ public class SlicedByteBuf extends AbstractDerivedByteBuf {
     @Override
     public ByteBuffer internalNioBuffer(int index, int length) {
         checkIndex(index, length);
-        return buffer.internalNioBuffer(index + adjustment, length);
+        return buffer.internalNioBuffer(index + adjustment, length).duplicate();
     }
 
     @Override

buffer/src/main/java/io/netty/buffer/SwappedByteBuf.java

@@ -741,8 +741,7 @@ public final class SwappedByteBuf extends ByteBuf {
 
     @Override
     public ByteBuffer internalNioBuffer(int index, int length) {
-        // Do not mess with the internal buffer's byte order.
-        return buf.nioBuffer(index, length).order(order);
+        return buf.internalNioBuffer(index, length).duplicate().order(order);
     }
 
     @Override