Stabilizing Celery / SQL Lab (#2981)

* upgrade celery to 4.0.2
* using Redis for unit tests (sqla broker not supported in Celery 4)
* Setting Celery's soft_time_limit based on `SQLLAB_ASYNC_TIME_LIMIT_SEC` config
* Better error handling in async tasks
* Better statsd logging in async tasks
* show [pending/running] query status in Results tab
* systematically using sqla NullPool on worker (async) to limit number
  of database connections

superset/assets/javascripts/SqlLab/actions.js

@@ -155,7 +155,7 @@ export function runQuery(query) {
         } else if (msg === null) {
           msg = `[${textStatus}] ${errorThrown}`;
         }
-        if (msg.indexOf('The CSRF token is missing') > 0) {
+        if (msg.indexOf('CSRF token') > 0) {
           msg = 'Your session timed out, please refresh your page and try again.';
         }
         dispatch(queryFailed(query, msg));