Netty4

Add support for netty4 communication module in 2.5.6 version of dubbo, enabled as follows:

provider：

<dubbo:protocol server="netty4" />

or

<dubbo:provider server="netty4" />

consumer：

<dubbo:consumer client="netty4" />

NOTES
1. If provider need to use different communication layer framework for different protocols , please configure multiple protocols separately. 2. consumer configuration as follow：

<dubbo:consumer client="netty">
  <dubbo:reference />
</dubbo:consumer>

<dubbo:consumer client="netty4">
  <dubbo:reference />
</dubbo:consumer>

Next we will continue to do something： 1. We will provide a reference data on the performance test indicators and performance test comparison with the version of netty 3.