RFR: 8044131: Restructure sjavac client / server protocol code

[Andreas Lundblad]

Hi compiler-dev,

Please review this patch which addresses part of JDK-8044131.

- Description:
The protocol code needs to be cleaned up. Currently the protocol code is entangled with the code that invokes javac (see CompilerThread.java). In order to replace the protocol code with something better it has to be factored out and separated from the compilation code. This is precisely what this patch does. It also hides the compilation logic behind a new interface (see JavacService.java) which will make the code more flexible. We will for instance be able to easily choose between an in-VM and a server/client implementation based on the background={true,fase} setting.

Be aware that this patch is part 1 of 2. Further cleanups ahead. CompilerThread and CompilerPool seems to dissapear for instance.

- Link to webrev:
http://cr.openjdk.java.net/~alundblad/8044131

- Links to bug report:
https://bugs.openjdk.java.net/browse/JDK-8044131

-- Andreas