Run importers in Parallel

[ziadhany]

Instead of running ./manage.py import --all in sequence We can use multiprocessing and try to use all CPU power.

suggestions:

• we can use Celery and schedule tasks and run easily importers in parallel (another benefit: we can run periodic Tasks and make importers synchronize )

• we can make it simple and use some code from scancode.

this could be a solution for #833

[ziadhany]

While I was trying to run importers using threads I couldn't notes any improvement ! https://stackoverflow.com/questions/31062124/multiple-db-inserts-with-django-performance-is-not-increased-by-parallel-threads