RFR 8131029: JShell: recover from VMConnection launch failure

[Robert Field]

Please review --

Attempt recovery from launch failure caused by either rare intermittent 
JDI failures or bad configurations by using an alternative connection 
and process-creation mechanism.
Necessitated a clean-up of the JShell JDI code: removed JDIEnv which 
provided no useful abstraction, coalescing the JDI connection 
functionality in JDIConnection;
remove unused and commented-out code from that class.
Add a meta-ExecutionControl class, FailOverExecutionControl, which 
cycles through ExecutionControl instances until one starts.

Bug:
     https://bugs.openjdk.java.net/browse/JDK-8131029

Webrev:
     http://cr.openjdk.java.net/~rfield/8131029v0.webrev/

Thanks,
Robert