Refactor: Cluster package

[justxuewei]

What would you like to be added:

• Move directory, route package to the root of the project;
• Remove cluster_filter from cluster like filter implementation;
• Move auxiliary packages, e.g. loadbalance, to cluster/addons

Why is this needed:

Cluster package of Dubbo-Go is too large and complex to develop new features, for example, directory not belongs to the cluster package, cuz it's used by both cluster and registry. After refactored, the cluster will only contains implementations about cluster.