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/components/QueryStateLabel.jsx

@@ -0,0 +1,19 @@
+import React from 'react';
+import PropTypes from 'prop-types';
+import { Label } from 'react-bootstrap';
+
+import { STATE_BSSTYLE_MAP } from '../constants';
+
+const propTypes = {
+  query: PropTypes.object.isRequired,
+};
+
+export default function QueryStateLabel({ query }) {
+  const bsStyle = STATE_BSSTYLE_MAP[query.state];
+  return (
+    <Label className="m-r-3" bsStyle={bsStyle}>
+      {query.state}
+    </Label>
+  );
+}
+QueryStateLabel.propTypes = propTypes;