[fix] deviceconnection_unique_together fails due to duplicate objects

[Dhanus3133]

Fixes #846

[coveralls]

coverage: 98.75%. remained the same
when pulling ad99f7b on Dhanus3133:bug/deviceconnection_unique_together
into f0bdf03 on openwisp:master.

[nemesifier]

It works on sqlite but as I feared, on PostgreSQL it throws this error:

django.db.utils.OperationalError: cannot ALTER TABLE "connection_deviceconnection" because it has pending trigger events

We have found out in prod that performing data migrations and schema migrations in the same migration class doesn't work well on Postgres.

Setting the atomic attribute to False has worked in my tests but it's, if anything goes wrong we will run into serious database consistency issues, it's not worth the risk.

The only reliable solution is to split the migration files:

• the first migration file should do the data migration
• the second data migration will perform the schema change

It will cause a bit of pain to whoever is using the dev version, because they will likely have already executed the existing migration, and running these new migrations will result into an error, but fixing that is as easy as running ./manage.py migrate connections --fake and I don't see any safer solution.

[Dhanus3133]

@nemesifier can you review it again