Enabling caching after sync doesn't add uncached documents until a folder's items change

[raucao]

When an app has an uncached folder (using the default SEEN strategy), syncs it, and potentially adds documents to it, then enabling full caching (ALL strategy) for the SEEN-synced folder should add all missing documents to the local cache.

Currently, it will only sync missing documents once there's a change to the folder items, while the missing documents are already present in the local itemsMap before then.