Shutdown Doc worker when it is not considered as available in Redis #831 (#856)

* Shutdown Doc worker when it is not considered as available in Redis
* Use isAffirmative for GRIST_MANAGED_WORKERS
* Upgrade Sinon for the tests
* Run Smoke test with pages in English
* Add logic in /status endpoint

stubs/app/server/declarations.d.ts

@@ -28,6 +28,7 @@ declare module "redis" {
   function createClient(url?: string): RedisClient;
 
   class RedisClient {
+    public readonly connected: boolean;
     public eval(args: any[], callback?: (err: Error | null, res: any) => void): any;
 
     public subscribe(channel: string): void;