stats: change update interval

Updating every 200 msec wastes battery; update every 950 msec instead.
Factor out write_qr() method

Pull the QR writing code out of the public_address property, and log
when it gets called.
CoinBank: start logging first thing

Start logging before anything else, and log the fact that we're building
a CoinBank.
Fix missing QR code for set address

If the donation address is set in the config file, and we're configured
to generate QR codes, then generate the code for the configured address.