fix potential blocking of NettyTransport connect and disconnect methods

Currently, in NettyTransport the locks for connecting and disconnecting channels are stored in a ConcurrentMap that has 500 entries. A tread can acquire a lock for an id and the lock returned is chosen on a hash function computed from the id.
Unfortunately, a collision of two ids can cause a deadlock as follows:

**Scenario**: one master (no data), one datanode (only data node)

DiscoveryNode id of master is X
DiscoveryNode id of datanode is Y

Both X and Y cause the same lock to be returned by NettyTransport#connectLock()

Both are up and running, all is fine until master stops.

**Thread 1**: The master fault detection of the datanode is notified (onNodeDisconnected()), which in turn leads the node to try and reconnect to master via the callstack titled "Thread 1" below.

-> connectToNode() is called and lock for X is acquired. The method waits for 45s for the cannels to reconnect.

Furthermore, Thread 1 holds the NettyTransport#masterNodeMutex.

**Thread 2**: The connection fails with an exception (connection refused, see callstack below), because the master shut down already. The exception is handled in NettyTransport#exceptionCaught which calls NettyTransport#disconnectFromNodeChannel. This method acquires the lock for Y (see Thread 2 below).

Now, if Y and X have two different locks, this would get the lock, disconnect the channels and notify thread 1. But since X and Y have the same locks, thread 2 is waiting for the lock thread 1 holds. thread 1 waits for thread 2 and then times out.

In this time, no thread can acquire the masterNodeMutex (held by thread 1), so the node can, for example, not stop.

This commit introduces a mechanism that assures unique locks for unique ids. This lock is not reentrant and therfore assures that threads can not end up in an infinite recursion (see Thread 3 below for an example on how a thread can aquire a lock twice).
While this is not a problem right now, it is potentially dangerous to have it that way, because the callstacks are complex as is and slight changes might cause unecpected recursions.

Thread 1
----

	owns: Object  (id=114)
	owns: Object  (id=118)
	waiting for: DefaultChannelFuture  (id=140)
	Object.wait(long) line: not available [native method]
	DefaultChannelFuture(Object).wait(long, int) line: 461
	DefaultChannelFuture.await0(long, boolean) line: 311
	DefaultChannelFuture.awaitUninterruptibly(long) line: 285
	NettyTransport.connectToChannels(NettyTransport$NodeChannels, DiscoveryNode) line: 672
	NettyTransport.connectToNode(DiscoveryNode, boolean) line: 609
	NettyTransport.connectToNode(DiscoveryNode) line: 579
	TransportService.connectToNode(DiscoveryNode) line: 129
	MasterFaultDetection.handleTransportDisconnect(DiscoveryNode) line: 195
	MasterFaultDetection.access$0(MasterFaultDetection, DiscoveryNode) line: 188
	MasterFaultDetection$FDConnectionListener.onNodeDisconnected(DiscoveryNode) line: 245
	TransportService$Adapter$2.run() line: 298
	EsThreadPoolExecutor(ThreadPoolExecutor).runWorker(ThreadPoolExecutor$Worker) line: 1145
	ThreadPoolExecutor$Worker.run() line: 615
	Thread.run() line: 724

Thread 2
-------

	waiting for: Object  (id=114)
	NettyTransport.disconnectFromNodeChannel(Channel, Throwable) line: 790
	NettyTransport.exceptionCaught(ChannelHandlerContext, ExceptionEvent) line: 495
	MessageChannelHandler.exceptionCaught(ChannelHandlerContext, ExceptionEvent) line: 228
	MessageChannelHandler(SimpleChannelUpstreamHandler).handleUpstream(ChannelHandlerContext, ChannelEvent) line: 112
	DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline$DefaultChannelHandlerContext, ChannelEvent) line: 564
	DefaultChannelPipeline$DefaultChannelHandlerContext.sendUpstream(ChannelEvent) line: 791
	SizeHeaderFrameDecoder(FrameDecoder).exceptionCaught(ChannelHandlerContext, ExceptionEvent) line: 377
	SizeHeaderFrameDecoder(SimpleChannelUpstreamHandler).handleUpstream(ChannelHandlerContext, ChannelEvent) line: 112
	DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline$DefaultChannelHandlerContext, ChannelEvent) line: 564
	DefaultChannelPipeline.sendUpstream(ChannelEvent) line: 559
	Channels.fireExceptionCaught(Channel, Throwable) line: 525
	NioClientBoss.processSelectedKeys(Set<SelectionKey>) line: 110
	NioClientBoss.process(Selector) line: 79
	NioClientBoss(AbstractNioSelector).run() line: 312
	NioClientBoss.run() line: 42
	ThreadRenamingRunnable.run() line: 108
	DeadLockProofWorker$1.run() line: 42
	ThreadPoolExecutor.runWorker(ThreadPoolExecutor$Worker) line: 1145
	ThreadPoolExecutor$Worker.run() line: 615
	Thread.run() line: 724

Thread 3
---------

        org.elasticsearch.transport.netty.NettyTransport.disconnectFromNode(NettyTransport.java:772)
        org.elasticsearch.transport.netty.NettyTransport.access$1200(NettyTransport.java:92)
        org.elasticsearch.transport.netty.NettyTransport$ChannelCloseListener.operationComplete(NettyTransport.java:830)
        org.jboss.netty.channel.DefaultChannelFuture.notifyListener(DefaultChannelFuture.java:427)
        org.jboss.netty.channel.DefaultChannelFuture.notifyListeners(DefaultChannelFuture.java:418)
        org.jboss.netty.channel.DefaultChannelFuture.setSuccess(DefaultChannelFuture.java:362)
        org.jboss.netty.channel.AbstractChannel$ChannelCloseFuture.setClosed(AbstractChannel.java:355)
        org.jboss.netty.channel.AbstractChannel.setClosed(AbstractChannel.java:185)
        org.jboss.netty.channel.socket.nio.AbstractNioChannel.setClosed(AbstractNioChannel.java:197)
        org.jboss.netty.channel.socket.nio.NioSocketChannel.setClosed(NioSocketChannel.java:84)
        org.jboss.netty.channel.socket.nio.AbstractNioWorker.close(AbstractNioWorker.java:357)
        org.jboss.netty.channel.socket.nio.NioClientSocketPipelineSink.eventSunk(NioClientSocketPipelineSink.java:58)
        org.jboss.netty.channel.Channels.close(Channels.java:812)
        org.jboss.netty.channel.AbstractChannel.close(AbstractChannel.java:197)
        org.elasticsearch.transport.netty.NettyTransport$NodeChannels.closeChannelsAndWait(NettyTransport.java:892)
        org.elasticsearch.transport.netty.NettyTransport$NodeChannels.close(NettyTransport.java:879)
        org.elasticsearch.transport.netty.NettyTransport.disconnectFromNode(NettyTransport.java:778)
        org.elasticsearch.transport.netty.NettyTransport.access$1200(NettyTransport.java:92)
        org.elasticsearch.transport.netty.NettyTransport$ChannelCloseListener.operationComplete(NettyTransport.java:830)
        org.jboss.netty.channel.DefaultChannelFuture.notifyListener(DefaultChannelFuture.java:427)
        org.jboss.netty.channel.DefaultChannelFuture.notifyListeners(DefaultChannelFuture.java:418)
        org.jboss.netty.channel.DefaultChannelFuture.setSuccess(DefaultChannelFuture.java:362)
        org.jboss.netty.channel.AbstractChannel$ChannelCloseFuture.setClosed(AbstractChannel.java:355)
        org.jboss.netty.channel.AbstractChannel.setClosed(AbstractChannel.java:185)
        org.jboss.netty.channel.socket.nio.AbstractNioChannel.setClosed(AbstractNioChannel.java:197)
        org.jboss.netty.channel.socket.nio.NioSocketChannel.setClosed(NioSocketChannel.java:84)
        org.jboss.netty.channel.socket.nio.AbstractNioWorker.close(AbstractNioWorker.java:357)
        org.jboss.netty.channel.socket.nio.NioWorker.read(NioWorker.java:93)
        org.jboss.netty.channel.socket.nio.AbstractNioWorker.process(AbstractNioWorker.java:109)
        org.jboss.netty.channel.socket.nio.AbstractNioSelector.run(AbstractNioSelector.java:312)
        org.jboss.netty.channel.socket.nio.AbstractNioWorker.run(AbstractNioWorker.java:90)
        org.jboss.netty.channel.socket.nio.NioWorker.run(NioWorker.java:178)
        java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        java.lang.Thread.run(Thread.java:724)