Switch from using FIFO to sockets

[propensive]

Bidirectional communication between the parent and child JVM would be very useful. One possible utility would be to reuse JVMs which don't have changed classpaths, which would make running tests faster.

The classpath checking would need to be done carefully. Addition of a classpath directory which contains classfiles that have already been added to the classpath should trigger the launch of a new, separate JVM, since it's not easy to unload an old class and replace it with a new one, while we would like to reuse classes if they haven't changed.