RFR 8160127, et. al.: JShell API: ExecutionControl implementation support

[Robert Field]

Responding to feedback that the originally produced API was huge, and 
the suggestion of remote-streamed symmetry/forwarding, I have rewritten 
the code.
The remote agent code was part of this rewrite and is now also included.
Please review --

Bugs:

     JShell API: extract abstract JDI and abstract streaming 
implementations of ExecutionControl
     https://bugs.openjdk.java.net/browse/JDK-8160127

     JShell API: Reorganize execution support code into 
jdk.jshell.execution (previously sent for review, and combined here)
     https://bugs.openjdk.java.net/browse/JDK-8159935

     JShell API: extract abstract streaming remote agent
     https://bugs.openjdk.java.net/browse/JDK-8160128

Parent bug:

     https://bugs.openjdk.java.net/browse/JDK-8159118

API:

     http://cr.openjdk.java.net/~rfield/jshell_8160127v1/

     Specifically --
http://cr.openjdk.java.net/~rfield/jshell_8160127v1/jdk/jshell/execution/package-summary.html
http://cr.openjdk.java.net/~rfield/jshell_8160127v1/jdk/jshell/spi/package-summary.html

Webrev:

     http://cr.openjdk.java.net/~rfield/8160127v1.webrev/


Thanks,
Robert