(core) Fix an insidious bug in RefCountMap, manifesting as JS errors some time after import.

Summary:
After an import from inside a document, one minute later, an important
QuerySet would get disposed, leaving the view section in a bad state,
and manifesting as JS errors on subsequent operations. (Might not
*always* happen because switching pages would prevent it from
manifesting, I think.)

Bad state that I've seen after transforms is probably explainable as
this bug, which is unrelated. Reproduction was hard because who knew one
had to wait a minute?!

Test Plan:
Added a unittest for the fix in QuerySet, and a browser test that
fails without the fix (JS errors, bad state), and passes with.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2653

app/client/models/QuerySet.ts

@@ -98,6 +98,12 @@ export class QuerySetManager extends Disposable {
   public purgeKey(queryKey: string) {
     this._queryMap.purgeKey(queryKey);
   }
+
+  // For testing: set gracePeriodMs, returning the previous value.
+  public testSetGracePeriodMs(ms: number): number {
+    return this._queryMap.testSetGracePeriodMs(ms);
+  }
+
 }
 
 /**