Retrieve list of nodes via DNS

[gabrielbosio]

Related to this issue: #11

Based on https://eips.ethereum.org/EIPS/eip-1459#client-protocol
It returns the node records for a specific network, without verifying them.

Known issue: Sometimes, when calling EthClient.NodesList.update_using_dns <testnet> it throws

** (exit) exited in: GenServer.call(EthClient.NodesList, {:update_using_dns, :rinkeby}, 20000)
    ** (EXIT) an exception was raised:
        ** (UndefinedFunctionError) function Socket.Error.exception/1 is undefined or private
            (socket 0.3.13) Socket.Error.exception([reason: :emfile])
            (socket 0.3.13) lib/socket/udp.ex:72: Socket.UDP.open!/1
            (dns 2.3.0) lib/dns.ex:72: DNS.query/4
            (dns 2.3.0) lib/dns.ex:23: DNS.resolve/4
            (eth_client 0.1.0) lib/eth_client/nodes_list/dns.ex:70: EthClient.NodesList.DNS.wait_to_resolve/2
            (eth_client 0.1.0) lib/eth_client/nodes_list/dns.ex:37: EthClient.NodesList.DNS.get_children/3
            (elixir 1.13.2) lib/task/supervised.ex:89: Task.Supervised.invoke_mfa/2
            (elixir 1.13.2) lib/task/supervised.ex:34: Task.Supervised.reply/4
    (elixir 1.13.2) lib/gen_server.ex:1030: GenServer.call/3

To check if all the connections are used, the DNS library should throw an exception and NodesList.DNS should catch it. But, in this case, it seems that an external library Socket is breaking.

Usage

# Get Goerli nodes using DNS
EthClient.NodesList.update_using_dns(:goerli)