Use solo pool for Celery workers

(Hopefully) achieve true single-threaded behaviour from a Celery worker by not having it use a threadpool at all. This should make it such that #workers == #concurrent_workflows

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