rule_list: should we consider a peer should be leader and follower in 2 rules in `checkApplyRules`

[Shirly]

Bug Report

when we try to add new placement-rule into the cluster, we would use the following function to check whether the new rule has conflictions with some exist rules.

pd/pkg/schedule/placement/rule_list.go

Lines 27 to 46 in fca469c

 func checkApplyRules(rules []*Rule) error { 
 	// check raft constraint 
 	// one and only one leader 
 	leaderCount := 0 
 	voterCount := 0 
 	for _, rule := range rules { 
 		if rule.Role == Leader { 
 			leaderCount += rule.Count 
 		} else if rule.Role == Voter { 
 			voterCount += rule.Count 
 		} 
 		if leaderCount > 1 { 
 			return errors.New("multiple leader replicas") 
 		} 
 	} 
 	if (leaderCount + voterCount) < 1 { 
 		return errors.New("needs at least one leader or voter") 
 	} 
 	return nil 
 } 

)

however, the above function miss the following corner case, which would save conflict placement-rule in the system:

both zone has only 1 tikv instance and the rule defines as the following:
leader should in tikv1, follower should in tikv1, for example:

 CREATE PLACEMENT POLICY leader_follower_in_1 LEADER_CONSTRAINTS="[+zone=bj1]" FOLLOWER_CONSTRAINTS="{+zone=bj1: 1,+zone=bj3: 1}"; 

the above function will return success, while we could never make the replica on tikv1 meet these 2 rules at the same time.