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/spec/javascripts/sqllab/QueryStateLabel_spec.jsx

@@ -0,0 +1,29 @@
+import React from 'react';
+import { Label } from 'react-bootstrap';
+import { shallow } from 'enzyme';
+import { describe, it } from 'mocha';
+import { expect } from 'chai';
+
+import QueryStateLabel from '../../../javascripts/SqlLab/components/QueryStateLabel';
+
+describe('SavedQuery', () => {
+  const mockedProps = {
+    query: {
+      state: 'running',
+    },
+  };
+  it('is valid', () => {
+    expect(
+      React.isValidElement(<QueryStateLabel />),
+    ).to.equal(true);
+  });
+  it('is valid with props', () => {
+    expect(
+      React.isValidElement(<QueryStateLabel {...mockedProps} />),
+    ).to.equal(true);
+  });
+  it('has an Overlay and a Popover', () => {
+    const wrapper = shallow(<QueryStateLabel {...mockedProps} />);
+    expect(wrapper.find(Label)).to.have.length(1);
+  });
+});