make backups work; make cold storage address optional

We can add that as a separate feature. My use case looks like this:

The txbits wallet actor tells bitcoind to back itself up whenever it forces bitcoind to generate new keys. I have another script that runs periodically and checks if the backup file exists. If it exists, it backs it up with a timestamp and deletes it. If the backup script doesn't check often enough bitcoind will just overwrite the backup with a more recent version.

Saving backups as multiple files with a new timestamp every time makes sense if you are doing your backup over a network mounted filesystem, but in any other case, your backup system can just pick up the file when it appears. We can add that feature, but it should be separate from these simple backups.