Fixes an import error when using S3/Minio with no redis

Error is caused due to these steps:
- File is uploaded to Home server and attempts to import
- Import ends up in `claimDocument` in `HostedStorageManager`
- Tries to read doc metadata from DocWorkerMap, gets 'unknown' as md5 hash
- Thinks local doc is out of date and erases it.
- Downloads a non-existent file from S3, so import fails as it has no data.

This fixes it by checking for DummyDocWorker's special 'unknown' MD5, forcing an S3 check.

app/gen-server/lib/DocWorkerMap.ts

@@ -580,8 +580,10 @@ let dummyDocWorkerMap: DummyDocWorkerMap|null = null;
 
 export function getDocWorkerMap(): IDocWorkerMap {
   if (process.env.REDIS_URL) {
+    log.info("Creating Redis-based DocWorker");
     return new DocWorkerMap();
   } else {
+    log.info("Creating local/dummy DocWorker");
     dummyDocWorkerMap = dummyDocWorkerMap || new DummyDocWorkerMap();
     return dummyDocWorkerMap;
   }

app/server/lib/DocManager.ts

@@ -650,6 +650,7 @@ export class DocManager extends EventEmitter {
         throw new Error('Grist docs must be uploaded individually');
       }
       const first = uploadInfo.files[0].origName;
+      log.debug(`DocManager._doImportDoc: Received doc with name ${first}`);
       const ext = extname(first);
       const basename = path.basename(first, ext).trim() || "Untitled upload";
       let id: string;
@@ -674,6 +675,7 @@ export class DocManager extends EventEmitter {
       }
       await options.register?.(id, basename);
       if (ext === '.grist') {
+        log.debug(`DocManager._doImportDoc: Importing .grist doc`);
         // If the import is a grist file, copy it to the docs directory.
         // TODO: We should be skeptical of the upload file to close a possible
         // security vulnerability. See https://phab.getgrist.com/T457.

app/server/lib/HostedStorageManager.ts

@@ -639,7 +639,7 @@ export class HostedStorageManager implements IDocStorageManager {
 
       const existsLocally = await fse.pathExists(this.getPath(docName));
       if (existsLocally) {
-        if (!docStatus.docMD5 || docStatus.docMD5 === DELETED_TOKEN) {
+        if (!docStatus.docMD5 || docStatus.docMD5 === DELETED_TOKEN || docStatus.docMD5 === 'unknown') {
           // New doc appears to already exist, but may not exist in S3.
           // Let's check.
           const head = await this._ext.head(docName);