Fix local_store.Store.lastSyncTimestamps() when store is empty

Make readItemIndex() always return a valid object rather than
null if lastSyncTimestamps() is invoked before any items
have been saved.
Fix dismissal of menu controls in Chrome ext

In the Chrome extension 'document' refers to the background
view's document, not the document which app UI has been
rendered into.

Use the document owning the menu DOM node instead.
Add a couple of additional sync logging statements