Make JobExecutor implement java.lang.AutoCloseable

[fmbenhassine]

JobExecutor can implement java.lang.AutoCloseable to be able to use it in a try-with-resources bloc and free the user from having to close it manually (and prevent him/her from forgetting to close it):

Job job = ..;
try (JobExecutor jobExecutor = new JobExecutor()) {
    jobExecutor.execute(job);
}