Bootstrap thread safety

Changed options and attrs from LinkedHashMap to ConcurrentHashMap to
avoid a possible ConcurrentModificationException if a thread was
adding/removing options/attrs while another was calling Bootstrap.init()
or Bootstrap.duplicate()

transport/src/main/java/io/netty/bootstrap/AbstractBootstrap.java

@@ -28,8 +28,8 @@
 import java.net.InetAddress;
 import java.net.InetSocketAddress;
 import java.net.SocketAddress;
-import java.util.LinkedHashMap;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 /**
  * {@link AbstractBootstrap} is a helper class that makes it easy to bootstrap a {@link Channel}. It support
@@ -40,8 +40,8 @@ public abstract class AbstractBootstrap<B extends AbstractBootstrap<?>> {
     private EventLoopGroup group;
     private ChannelFactory factory;
     private SocketAddress localAddress;
-    private final Map<ChannelOption<?>, Object> options = new LinkedHashMap<ChannelOption<?>, Object>();
-    private final Map<AttributeKey<?>, Object> attrs = new LinkedHashMap<AttributeKey<?>, Object>();
+    private final Map<ChannelOption<?>, Object> options = new ConcurrentHashMap<ChannelOption<?>, Object>();
+    private final Map<AttributeKey<?>, Object> attrs = new ConcurrentHashMap<AttributeKey<?>, Object>();
     private ChannelHandler handler;
 
     /**