Improve db handling

See merge request !240

This will avoid some concurrency problem especially with SQLite.

Related to the use of the multiprocessing module.

See:
https://code.djangoproject.com/ticket/20562

This avoids database creation errors as other applications depends
on contenttypes.

Delete cached files

See merge request !239

This will go through all results and inputs of all experiments
that are using this cache.