Reserve highest job ID bit for intercluster jobs

In order to support inter-cluster job dependencies, the MaxJobID
configuration parameter default value has been reduced from 4,294,901,760
to 2,147,418,112 and it's maximum value is now 2,147,463,647.
ANY JOBS WITH A JOB ID ABOVE 2,147,463,647 WILL BE PURGED WHEN SLURM IS
UPGRADED FROM AN OLDER VERSION!