Make Bootstrap and ServerBootstrap fully overridable

Related: #2034

Motivation:

Some users want to mock Bootstrap (or ServerBootstrap), and thus they
should not be final but be fully overridable and extensible.

Modifications:

Remove finals wherever possible

Result:

@daschl is happy.

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

@@ -382,7 +382,7 @@ public abstract class AbstractBootstrap<B extends AbstractBootstrap<B, C>, C ext
     /**
      * Return the configured {@link EventLoopGroup} or {@code null} if non is configured yet.
      */
-    public final EventLoopGroup group() {
+    public EventLoopGroup group() {
         return group;
     }
 

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

@@ -38,7 +38,7 @@ import java.util.Map.Entry;
  * <p>The {@link #bind()} methods are useful in combination with connectionless transports such as datagram (UDP).
  * For regular TCP connections, please use the provided {@link #connect()} methods.</p>
  */
-public final class Bootstrap extends AbstractBootstrap<Bootstrap, Channel> {
+public class Bootstrap extends AbstractBootstrap<Bootstrap, Channel> {
 
     private static final InternalLogger logger = InternalLoggerFactory.getInstance(Bootstrap.class);
 

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

@@ -42,7 +42,7 @@ import java.util.concurrent.TimeUnit;
  * {@link Bootstrap} sub-class which allows easy bootstrap of {@link ServerChannel}
  *
  */
-public final class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {
+public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {
 
     private static final InternalLogger logger = InternalLoggerFactory.getInstance(ServerBootstrap.class);