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

if the machine didn't have resolvconf before (my box didn't after an upgrade from Ubuntu 13.xx), make sure it has it now and archive any old resolv.conf since it should now only list 127.0.0.1 for bind9

migrate the migration state from MIGRATIONID in /etc/mailinabox.conf to STORAGE_ROOT/mailinabox.version so that the data format of STORAGE_ROOT is stored in the directory itself