Consensus heartbeat statistics

Currently, all heartbeat samples are cached above the consensus layer of ConfigNode, which connotes that all load statistics will be discarded as long as the leader of ConfigNode is switched. Obviously, the following kinds of loadStatistics are worth to consensus:

• The status of cluster Nodes and RegionGroups
• The leader of RegionGroup
• The loadScore of cluster Nodes and RegionGroups(will be implemented in the future)

Therefore, I refactor the framework of cluster heartbeat related threads, and you can find the detailed desgin in the LoadBalancing Framework doc.