Deal with attempts=NULL
JobSoftwareRequirement {max,min}_version rename

All other columns in the database model that reference an id column of
another table have an "_id" suffix. Add this suffix to min_version and
max_version, too.
Lazy loading for requirements for jobs
Version relations in JobSoftwareRequirement
Disallow setting job timestamps manually
Remove unused variable
Don't complain about missing keys where disallowed
Minor cosmetic fixes in the source
Allow requirements when submitting jobs