public void setReadOnly(boolean readOnly) {
        this.readOnly = readOnly;
    }
Set the message to be readonly,but metamorphosis client and server could
modify message's id,flag,partition.The readonly state is transient,it
will not be persist in broker.

@since 1.4.4
@param readOnly
metamorphosis-commons/src/main/java/com/taobao/metamorphosis/Message.java