Unique attribute key name clash

[NiteshKant]

ClientRequestResponseConverter uses statically created AttributeKey instances here which will cause issues in few cases:

• If the handler is loaded in multiple classloaders.
• If the handler is in a shaded rxnetty jar.

The error stack will be something like this:

2015-05-04 21:19:11 WARN  ChannelInitializer:151 rxnetty-nio-eventloop-7-1 - Failed to initialize a channel. Closing: [id: 0x9e4943d9]
java.lang.ExceptionInInitializerError
    at iep.io.reactivex.netty.protocol.http.client.ClientRequiredConfigurator.configureNewPipeline(ClientRequiredConfigurator.java:42)
    at iep.io.reactivex.netty.pipeline.PipelineConfiguratorComposite.configureNewPipeline(PipelineConfiguratorComposite.java:55)
    at iep.io.reactivex.netty.pipeline.PipelineConfiguratorComposite.configureNewPipeline(PipelineConfiguratorComposite.java:55)
    at iep.io.reactivex.netty.client.RxClientImpl$2.initChannel(RxClientImpl.java:127)
    at io.netty.channel.ChannelInitializer.channelRegistered(ChannelInitializer.java:69)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRegistered(AbstractChannelHandlerContext.java:158)
    at io.netty.channel.AbstractChannelHandlerContext.fireChannelRegistered(AbstractChannelHandlerContext.java:144)
    at io.netty.channel.DefaultChannelPipeline.fireChannelRegistered(DefaultChannelPipeline.java:732)
    at io.netty.channel.AbstractChannel$AbstractUnsafe.register0(AbstractChannel.java:443)
    at io.netty.channel.AbstractChannel$AbstractUnsafe.access$100(AbstractChannel.java:375)
    at io.netty.channel.AbstractChannel$AbstractUnsafe$1.run(AbstractChannel.java:419)
    at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:370)
    at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:357)
    at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:116)
    at io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:137)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.IllegalArgumentException: 'rxnetty_http_conn_keep_alive_timeout_millis' is already in use
    at io.netty.util.UniqueName.<init>(UniqueName.java:53)
    at io.netty.util.AttributeKey.<init>(AttributeKey.java:47)
    at io.netty.util.AttributeKey.valueOf(AttributeKey.java:39)
    at iep.io.reactivex.netty.protocol.http.client.ClientRequestResponseConverter.<clinit>(ClientRequestResponseConverter.java:84)