ExecutorService configuration improvment

[christophe-w]

When configuring an ExecutorService in the parallelj.xml configuration file, we can now specify a new attribute in the xml tag "executor-service": type="INSTANCE"

When this new attribute is set to the value "INSTANCE", ParallelJ will use an ExecutorService instance for each instance of the Program.
It's usefull for exemple if a ForEachLoop is linked to a SubProgram.
An new ExecutorService will be created and used for each instance of the SubProgram. In this case, if the ExecutorService only use on Thread, this allows to managed transaction by SubProgram instance.