Focus on work of next release 2.6.6

[Minxuan Zhuang]

Dubbo2.6.5 has been released, I want to take responsibility to release
2.6.6. Maybe it will be released in middle January， 2019.
here is the milestone for 2.6.6, and I will move proper existing issues
into it.:
https://github.com/apache/incubator-dubbo/milestone/22
If anyone fix an issue or add new feature, you can add a 2.6.6 milestone