upgrade.CONFIG_VERSION

The [tether] version this code writes and expects. tether upgrade brings

upgrade.CONFIG_VERSION=5

older datasets forward (see tether.upgrade; removed at 0.1.0). 5 is 0.1.0b4’s: what 0.1.0b3 recorded under identities and states that changed is carried forward, and a 0.1.0b3 refuses the dataset from then on.