(core) start reconciling forking with granular access

Summary:
This allows a fork to be made by a user if:
 * That user is an owner of the document being forked, or
 * That user has full read access to the document being forked.

The bulk of the diff is reorganization of how forking is done.  ActiveDoc.fork is now responsible for creating a fork, not just a docId/urlId for the fork. Since fork creation should not be limited to the doc worker hosting the trunk, a helper endpoint is added for placing the fork.

The change required sanitizing worker allocation a bit, and allowed session knowledge to be removed from HostedStorageManager.

Test Plan: Added test; existing tests pass.

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2700

app/server/lib/DocSnapshots.ts

@@ -121,6 +121,13 @@ export class DocSnapshotInventory implements IInventory {
     });
   }
 
+  /**
+   * Return true if document inventory does not need to be saved and is not in flux.
+   */
+  public isSaved(key: string) {
+    return !this._needFlush.has(key) && !this._mutex.isLocked(key);
+  }
+
   /**
    * Add a new snapshot of a document to the existing inventory.  A prevSnapshotId may
    * be supplied as a cross-check.  It will be matched against the most recent