PDI-12318 - NullPointerException running job with massive parallel Job/Transformation loads

Fix concurrency issue. Lists being modified concurrently (for parallel
jobs execution must be synchronized).
	modified:   engine/src/org/pentaho/di/job/Job.java
Merge pull request #694 from prakapenka/PDI-12318

PDI-12318 - NullPointerException running job with massive parallel Job/Transformation loads