Optimize _GetJobIDsUnlocked

Currently we sort the list of job queue files twice (once in
utils.ListVisibleFiles with sort and then later with NiceSort). We apply
the _RE_JOB_FILE regular expression twice (once in _ListJobFiles and
once in _ExtractJobID). This simplifies the code a little, and a couple
of functions performing basically the same job are collapsed.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>