`AlreadyWaiting` exception during initial discovery.

[pipermerriam]

What is wrong?

   ERROR  05-23 16:16:10  base_events  Task exception was never retrieved
future: <Task finished coro=<PeerPool.lookup_random_node() done, defined at /Users/piper/sites/py-evm/p2p/peer.py:741> exception=AlreadyWaiting("There's another coroutine waiting for a neighbours packet from <Node(0x9124@148.251.19.50)>",)>
Traceback (most recent call last):
  File "/Users/piper/sites/py-evm/p2p/peer.py", line 745, in lookup_random_node
    await self.discovery.lookup_random(self.cancel_token)
  File "/Users/piper/sites/py-evm/p2p/discovery.py", line 93, in lookup_random
    return await self.kademlia.lookup_random(self.cancel_token.chain(cancel_token))
  File "/Users/piper/sites/py-evm/p2p/kademlia.py", line 575, in lookup_random
    return await self.lookup(random.randint(0, k_max_node_id), cancel_token)
  File "/Users/piper/sites/py-evm/p2p/kademlia.py", line 565, in lookup
    *[_find_node(node_id, n) for n in nodes_to_ask])
  File "/Users/piper/sites/py-evm/p2p/kademlia.py", line 541, in _find_node
    candidates = await self.wait_neighbours(remote, cancel_token)
  File "/Users/piper/sites/py-evm/p2p/kademlia.py", line 461, in wait_neighbours
    "There's another coroutine waiting for a neighbours packet from {}".format(remote))
p2p.kademlia.AlreadyWaiting: There's another coroutine waiting for a neighbours packet from <Node(0x9124@148.251.19.50)>

looks like a race condition of some kind which results in stacktrace being dumped to terminal.

[pipermerriam]

Fixed in #765