public void setPriority(int priority) {
        this.priority = priority;
        this.str = null;
    }
Parse a peer from string in the format of "ip:port:idx",
returns null if fail to parse.

@param s input string with the format of "ip:port:idx"
@return parsed peer
jraft-core/src/main/java/com/alipay/sofa/jraft/entity/PeerId.java