(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/common/KeyedOps.ts

@@ -99,6 +99,7 @@ export class KeyedOps {
       await status.promise;
       return;
     }
+    if (!this._changed.has(key)) { return; }
     const callback = new Promise((resolve) => {
       status.callbacks.push(resolve);
     });