Fix message size check (#577)
* Fix message size check
The message size check was added in the wrong place and so also affecting MDNS (where formally such messages are also too long but accepted potentially).
For sending the specification states:
```
Messages sent over TCP or BTP over BLE transports MAY exceed the message size limit if both nodes are capable of supporting larger message sizes.
```
so I just added logging for now. Should not happen that we generate too big messages.
* Add missing import
