Fix NPE when initializing an accepted socket in NettyTransport.

NettyTransport's ChannelPipelineFactory uses the instance variable
serverOpenChannels in order to create sockets. However, this instance variable
is set to null when stoping the netty transport, so if the transport tries to
stop and to initialize a socket at the same time you might hit the following
NullPointerException:

[2014-05-13 07:33:47,616][WARN ][netty.channel.socket.nio.AbstractNioSelector] Failed to initialize an accepted socket.
java.lang.NullPointerException: handler
	at org.jboss.netty.channel.DefaultChannelPipeline$DefaultChannelHandlerContext.<init>(DefaultChannelPipeline.java:725)
	at org.jboss.netty.channel.DefaultChannelPipeline.init(DefaultChannelPipeline.java:667)
	at org.jboss.netty.channel.DefaultChannelPipeline.addLast(DefaultChannelPipeline.java:96)
	at org.elasticsearch.transport.netty.NettyTransport$2.getPipeline(NettyTransport.java:327)
	at org.jboss.netty.channel.socket.nio.NioServerBoss.registerAcceptedChannel(NioServerBoss.java:134)
	at org.jboss.netty.channel.socket.nio.NioServerBoss.process(NioServerBoss.java:104)
	at org.jboss.netty.channel.socket.nio.AbstractNioSelector.run(AbstractNioSelector.java:318)
	at org.jboss.netty.channel.socket.nio.NioServerBoss.run(NioServerBoss.java:42)
	at org.jboss.netty.util.ThreadRenamingRunnable.run(ThreadRenamingRunnable.java:108)
	at org.jboss.netty.util.internal.DeadLockProofWorker$1.run(DeadLockProofWorker.java:42)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:745)

This fix ensures that the ChannelPipelineFactory always uses the channels that
have been used upon start, even if a stop request is issued concurrently.

Close #6144