Import job duplicate id fix

The current implementation of assigning id's to ImportingJobs allows the same id to be assigned to more than one concurrent job, due to the sum of System.currentTimeMillis() and the randomized value being the same.

This is moderately frequent under heavy load with many concurrent jobs and causes jobs to fail since they are missing in the jobs-collection.

The new implementation is thread-safe and guarantees Long.MAX_VALUE unique id's before wrapping around to 1.