Make jobtype name unique per parent jobqueue

[guidow]

Jobtype names should be unique per parent jobqueue instead of globally across the whole system. This could allow jobqueue structures like

project 1
+- lighting
project 2
+- lighting

Unfortunately, this isn't entirely trivial because of the special case of jobqueues on the top level, which is signified by jobqueue_id=NULL. Different SQL servers treat NULL values differently inside of unique constraints – PostgreSQL treats them as always different (which is what the SQL standard says, but not very helpful for us in this case), SQL Server treats them as equal. Dunno about MySQL or SQLite.

[opalmer]

Hmm, I'd really like to avoid having to write in different behaviors per db. I wonder if we should rethink the approach our approach to 'global' some? We're likely to have this same issue in other areas where we need a global state and overrides.