RFR: JDK-8186694: JShell: speed-up compilation by reusing compiler instances

[Jan Lahoda]

Hello,

When starting jshell like this (where "/tmp/script" is a file which has 
just "/exit"):
$ jshell PRINTING --execution local /tmp/script

It currently takes approx. 5-6s on my laptop. PRINTING will add quite a 
few startup snippets. Each of these snippets is processed separately and 
needs several instances of javac to be created to be processed correctly.

So, the idea in this patch is to reuse the javac instances multiple 
(many) times. This avoids initializing the javac instances just to 
process a very small ("single line") input.

This patch is based on the ReusableContext by Maurizio used in tests. 
The ReusableContext handling is wrapped in JavacTaskPool, which keeps 
the ReusableContext and issues JavacTask possibly based on a reused 
Context. The pool is not currently general purpose, but should work for 
tests for which ReusableContext worked and for JShell.

One tricky thing is that in JShell, it is necessary to prune everything 
in the "REPL" package when reusing the Context, as it is necessary to 
reload the correct/updated classes again from the file manager (this is 
achieved by additional cleaning in TaskFactory).

Overall, with this patch, the time to execute the above command is 
approx. 2.5s

Bug:
https://bugs.openjdk.java.net/browse/JDK-8186694

Webrev:
http://cr.openjdk.java.net/~jlahoda/8186694/webrev.00/

How does this look?

Thanks,
     Jan