Allow only specific actors defined as a sticky leader to become a leader

[ydash]

Allow only specific actors defined as a sticky leader to become a leader when RaftSettings.stickyLeaders is not empty.

• add API of withStickyLeaders
• add following features to Follower.scala and Candidate.scala
  • if stickyLeaders is not empty, only raft actors defined as a sticky leader can become Leader
  • if stickyLeaders is empty, all raft actors can become Leader
• add following feature to RaftActor.scala
  • notice warn log when creation of raft actor if stickyLeaders is not empty