DNS entity improvements

BindDnsServer rebinds, supports CNAME records and is better tested. 

@grkvlt Would be great if you could review this thoroughly.
Perhaps better to use an explicit mutex `Object` instead? Also, do we still need the synchronisation, since the sensor data is now pushed out only once changed, so there shouldn't be inconsistencies? It's really just enforcing serialisation of calls to `update()` now, right?