[WIP] Make uuid columns unique in sqla

[ltalirz]

• replace superfluous aiida.backends.sqlalchemy.models.utils.uuid_func with aiida.common.utils.get_new_uuid
• make uuid columns unique in sqla as well
  • @giovannipizzi Running the alembic command produced an empty migration, i.e. I copied the code from the corresponding migration for the uuid of DbNode. Not sure whether this is expected behavior
• methods to check for duplicate uuids now generalized to take table as argument
• upgrade prospector - prospector 1.1.5 finally works with pylint <2 again Pylint throws exception for unexpected keyword argument when running prospector. landscapeio/prospector#266

Todo:

• decide: do we need to check for duplicate uuids also for the other tables?
  I added the check now but do not yet provide a way to resolve the problem if there is one.
• (?) If we do need to handle the deduplication manually, understand what is needed here (I guess these classes are less complicated than Node)
• (?) Add test in sqla that there are no remaining migrations

[coveralls]

Coverage decreased (-0.01%) to 68.841% when pulling bb7e1df on ltalirz:uuid-unique-sqla into aa79072 on aiidateam:provenance_redesign.

[ltalirz]

closing for further rework