Dev/monetdb async connection

Created by: KFilippopolitis

Changelog:

1. Removed module aiopymonetdb.
2. Update connection to be a singleton and support call concurrently.
3. Added occ handling on the non select queries (Create, Drop, Alter, ...)
4. Updated test to run simultaneously to emulate the concurrent calls.
5. Bugfixes on tests.
6. Temporary disabled celery worker's parallelism due to a MonetDB bug.