PD graceful upgrade

[xiaojingchen]

pd graceful upgrade must evict leader to other member before upgrade the member.
the major logic of pd upgrade:

        for i = replicas - 1; i >= 0; i-- {
		if !canUpgrade(i) {
			continue
		}
		currentLeader := queryLeader()
		if pd[i].memberID != currentLeader {
			upgrade(i)
		} else {
			if i == replicas-1 {
				transferTo(0)
			} else {
				transferTo(replicas - 1)
			}
		}
	}

pd's leader api:

• query current leader : /pd/api/v1/leader
• transfer leader to special member : /pd/api/v1/leader/transfer/{memberName}