RFR:8219721: jcmd from earlier release will hang attaching to VM with JDK-8215622 applied

[臧琳]

Dear All, 
   May I ask your help to review the fix of compatibility issue introduced by JDK-8215622
   webrev:  http://cr.openjdk.java.net/~xiaofeya/8219721/webrev.01/
   Bug:  https://bugs.openjdk.java.net/browse/JDK-8219721

   FYI, this webrev is forked from the discussion https://mail.openjdk.java.net/pipermail/serviceability-dev/2019-February/027240.html.

Thanks,
Lin