Store files without caching locally

@skddc this should enable you to fix your image sharing app (even before the indexedDB adapter is stable). You're probably calling `client.storeFile` at some point. Just set the "cache" parameter to "false", then the file won't be cached locally. Make sure to disable sync of the image-directory entirely (`client.release('')`), or only use tree-only sync (`client.use('', true)`).