MacOS: possible data loss/corruption

Boltdb depends on os.File.Sync().

Go doesn't implement .Sync() correctly on MacOS in Go <= 1.11. A fix is in tree for 1.12. https://github.com/golang/go/issues/26650

bbolt also depends on .Sync() but there is a fix they could apply until 1.12 appears. https://github.com/etcd-io/bbolt/issues/124