backup: dont remove old increments because then we lose the backup history right before the last full backup, instead let them disappear along with full backups when a whole chain becomes very old

Doesn't seem like 2 minutes is a problem, but 4 minutes seems better. A little less bandwidth, possibly less battery usage (though we don't have evidence that's actually true), and the interval should be shorter than any peer timeouts that might occur due to inactivity

fixes #129