Use solo pool for Celery workers

start-worker.sh

@@ -4,4 +4,4 @@ set -o errexit
 set -o nounset
 
 cd /app
-python -m celery -A gso.worker worker --loglevel=info --concurrency=1
+python -m celery -A gso.worker worker --loglevel=info --concurrency=1 --pool=solo