(core) correctly delete pruned document versions

Summary:
After switch to using an inventory file, old document versions were
not in fact being pruned.  This corrects that and adds a test
that fails with the previous implementation.

The pruner was operating correctly, but was being applied to an
inventory list rather than s3 directly - and the inventory list
did not pass through version removals to s3.

This fix will leave a stock of undeleted versions that can
be eliminated by an external script (there are alternatives
but that seems simplest overall).

Test Plan: updated test

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2693

app/server/lib/DocSnapshots.ts

@@ -148,6 +148,7 @@ export class DocSnapshotInventory implements IInventory {
       if (oldIds.size > 0) {
         const results = current.filter(v => !oldIds.has(v.snapshotId));
         const fname = await this._getFilename(key);
+        await this._doc.remove(key, snapshotIds);
         await this._saveToFile(fname, results);
         this._needFlush.add(key);
       }