Use a common thread pool for all TimerJobInstances

From what I've seen(correct me if I'm wrong) there is, for each StatefulKnowledgeSession a TimeManager -> JDKTimerService instances pair that use a ScheduledThreadPoolExecutor of size one(default) for scheduling timer job instances.
Now the issues that I see are:

• not optimal resource utilization in case of large number of sessions
• number of sessions bounded by available memory for threads
• some performance penalty for concurrent jobs (I say some because all session ops are serial)

I provided also a PR with to be reviewed