Support multiple preferred leader

[Git-Yang]

1. Background
In the mixed deployment mode of namesrv and broker, we hope that the machine/node where namesrv and broker are mixed will not be selected as the leader, and other machines/nodes can be selected as the leader. At the same time, we hope to trigger the re-election as little as possible, so multiple nodes are set as the preferredLeader to reduce the occurrence of this situation.
2. Optimization

• Allow multiple nodes to be set as preferredLeader.