Flush S3 object files from SM cache into S3

As of now there is no way to tell StorageManager to flush the whole SM cache to S3 to make a consistent data snapshot.
This issue is a per-requisite for a consistent S3-based installation snapshots.
On success SM doesn't have any data in its cache that might take some time.
On failure SM must explicitly notify about an error and maybe set DBRM into readonly.