Consider adding NS records to point to gateways

[decentralgabe]

Currently there is no inter-gateway communication (#20). This leads to some practical issues, like historical state not being available across gateways unless the same DID is published to multiple gateways (burden on the client).

This makes it tough for anyone interacting with a DID to find its history, since they don't know which gateway(s) the DID was published to. To solve this problem we can make use of DNS NS records to point to a gateway, included in the packet stored in the DHT.

This could also work as another special TXT record that says something to effect of "I trust this gateway to be a source of truth for my document."

cc: @frankhinek @mistermoe @csuwildcat