(core) be careful when reassigning a doc to a worker it was on before

Summary:
Importing a .grist document is implemented in a somewhat clunky way, in a multi-worker setup.

 * First a random worker receives the upload, and updates Grist's various stores appropriately (database, redis, s3).
 * Then a random worker is assigned to serve the document.

If the worker serving the document fails, there is a chance the it will end up assigned to the worker that handled its upload. Currently the worker will misbehave in this case. This diff:

 * Ports a multi-worker test from test/home to run in test/s3, and adds a test simulating a bad scenario seen in the wild.
 * Fixes persistence of any existing document checksum in redis when a worker is assigned.
 * Adds a check when assigned a document to serve, and finding that document already cached locally. It isn't safe to rely only on the document checksum in redis, since that may have expired.
 * Explicitly claims the document on the uploading worker, so this situation becomes even less likely to arise.

Test Plan: added test

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3305

app/server/lib/ITestingHooks.ts

@@ -12,6 +12,7 @@ export interface ITestingHooks {
   closeDocs(): Promise<void>;
   setDocWorkerActivation(workerId: string, active: 'active'|'inactive'|'crash'): Promise<void>;
   flushAuthorizerCache(): Promise<void>;
+  flushDocs(): Promise<void>;
   getDocClientCounts(): Promise<Array<[string, number]>>;
   setActiveDocTimeout(seconds: number): Promise<number>;
   setDiscourseConnectVar(varName: string, value: string|null): Promise<string|null>;