DeadLetters from NetworkController

Sometimes the following record appears in logs:
```[akka://ergoref/user/networkController/$d] Message [scorex.core.network.message.Message] from Actor[akka://ergoref/user/networkController#-1129321538] to Actor[akka://ergoref/user/networkController/$d#1016405079] was not delivered. [2] dead letters encountered.```