JobID.forName() isn't strict enough

If you have a job running as job_201208221603_0003, and then try to kill a job passing id job_201208221603_003, it will kill job_201208221603_0003 because the last part of the JobID is parsed as an integer. We should make JobID.forName() stricter to prevent this and similar situations as the current behavior isn't so obvious.

More specifically, we shouldn't accept JobIDs if the last part of the JobID is:
- less than 4 characters (e.g. _003, _123)
- more than 4 characters and has a leading zero (e.g. _00003, _01234)