feat: replace gunicorn worker with gthread

The meinheld worker seems to be unmaintained and it doesn't work with
gunicorn's reload mechanism.

start.sh

@@ -2,7 +2,7 @@
 set -eu
 if [ ${MODE:-""} == "development" ]; then 
     if [ -f /app/requirements.txt ]; then pip install -r /app/requirements.txt; fi
-    exec gunicorn -k --reload egg:meinheld#gunicorn_worker -c "$GUNICORN_CONF" "$APP_MODULE"
+    exec gunicorn -k --reload gthread -c "$GUNICORN_CONF" "$APP_MODULE"
 else 
-    exec gunicorn -k egg:meinheld#gunicorn_worker -c "$GUNICORN_CONF" "$APP_MODULE"
+    exec gunicorn -k gthread -c "$GUNICORN_CONF" "$APP_MODULE"
 fi