Optimize element deletion using raw SQL

arkindex/documents/api/elements.py

arkindex/documents/deletion.py

arkindex/documents/management/commands/delete.py

arkindex/documents/models.py

arkindex/documents/signals.py

arkindex/documents/tests/consumers/test_corpus_consumer.py

arkindex/documents/tests/test_corpus.py

arkindex/documents/tests/test_destroy_elements.py

arkindex/documents/tests/test_retrieve_elements.py