Add WorkerActivity start time and average/min/max processing time

arkindex/documents/tests/test_destroy_elements.py

arkindex/process/api.py

arkindex/process/managers.py

arkindex/process/migrations/0062_workeractivity_started.py

arkindex/process/models.py

arkindex/process/serializers/workers.py

arkindex/process/tests/test_signals.py

arkindex/process/tests/test_workeractivity.py

arkindex/process/tests/test_workeractivity_stats.py

arkindex/sql_validation/workeractivity_bulk_insert.sql

...alidation/workeractivity_bulk_insert_no_configuration.sql