core/authority-discovery: Enable authorities to discover each other (#3452)

With the *authority-discovery* module an authoritative node makes itself
discoverable and is able to discover other authorities. Once discovered, a node
can directly connect to other authorities instead of multi-hop gossiping
information.

1. **Making itself discoverable**

    1. Retrieve its external addresses

    2. Adds its network peer id to the addresses

    3. Sign the above

    4. Put the signature and the addresses on the libp2p Kademlia DHT

2. **Discovering other authorities**

    1. Retrieve the current set of authorities

    2. Start DHT queries for the ids of the authorities

    3. Validate the signatures of the retrieved key value pairs

    4. Add the retrieved external addresses as ~reserved~ priority nodes to the
       peerset


* node/runtime: Add authority-discovery as session handler

The srml/authority-discovery module implements the OneSessionHandler in
order to keep its authority set in sync. This commit adds the module to
the set of session handlers.

* core/network: Make network worker return Dht events on poll

Instead of network worker implement the Future trait, have it implement
the Stream interface returning Dht events.

For now these events are ignored in build_network_future but will be
used by the core/authority-discovery module in subsequent commits.

* *: Add scaffolding and integration for core/authority-discovery module

* core/authority-discovery: Implement module logic itself