Begrudingly allow sqlite3_js_kvvfs_clear() to work for opened storage only for the...

Begrudingly allow sqlite3_js_kvvfs_clear() to work for opened storage only for the local/session storage cases and only (one of the demo apps reminds me) for backwards compatiblity. Likewise, the JsStorageDb.clearStorage() inherited method is now deprecated because it requires an opened db (but continues to work for those two stores), whereas its 'static' method of the same name is unaffected. Rename kvvfs.size() to kvvfs.estimateSize().

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