RFR 8173893: JShell: reduce memory leaks

[Robert Field]

Thanks Jan for tracking down the leaks!!!

Please review...

Bug:

     https://bugs.openjdk.java.net/browse/JDK-8173893

Webrev:

     http://cr.openjdk.java.net/~rfield/8173893v0.webrev/

Thanks,
Robert

:y