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/ResultSet.jsx

@@ -6,6 +6,7 @@ import shortid from 'shortid';
 import VisualizeModal from './VisualizeModal';
 import HighlightedSql from './HighlightedSql';
 import FilterableTable from '../../components/FilterableTable/FilterableTable';
+import QueryStateLabel from './QueryStateLabel';
 
 const propTypes = {
   actions: PropTypes.object,
@@ -165,6 +166,7 @@ export default class ResultSet extends React.PureComponent {
       return (
         <div>
           <img className="loading" alt="Loading..." src="/static/assets/images/loading.gif" />
+          <QueryStateLabel query={query} />
           {progressBar}
         </div>
       );