BackupProvider JSON-RPC: make getting QR code easier

[flub]

Currently a user of the JSON-RPC API has to guess when they can call the function to retrieve the QR code. They can hack around this by waiting for a specific IMEX progress event but that's not nice either.

The proposal would be make the get_backup_qr() method blocking if there is currently a call to provide_backup() and once the QR code is available it would return. If get_backup_qr() is called without a current provide_backup() call it will error immediately.