(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/DocManager.ts

@@ -530,7 +530,13 @@ export class DocManager extends EventEmitter {
         const srcDocPath = uploadInfo.files[0].absPath;
         await checkAllegedGristDoc(docSession, srcDocPath);
         await docUtils.copyFile(srcDocPath, docPath);
-        await this.storageManager.addToStorage(docName);
+        // Go ahead and claim this document. If we wanted to serve it
+        // from a potentially different worker, we'd call addToStorage(docName)
+        // instead (we used to do this). The upload should already be happening
+        // on a randomly assigned worker due to the special treatment of the
+        // 'import' assignmentId.
+        await this.storageManager.prepareLocalDoc(docName);
+        this.storageManager.markAsChanged(docName, 'edit');
         return {title: basename, id: docName};
       } else {
         const doc = await this.createNewEmptyDoc(docSession, id);