[REFACTOR] updates to DHT internals

[justheuristic]

extracted from #98

• Can we get rid of return_futures=True option in node.get/store/run_coro ?
  • replace return_future -> wait=True
  • or 2nd method get_async / store_async
• rewrite test_dht_protocol, test_empty_table and test_dht_node to use await instead of loop.run_until_complete
• make DHT resistant to KeyboardInterrupt when they are running interactively?
  • problem symptom: you're running in jupyter, you press ctrl+C and it kills all dht daemons
  • applies to DecentralizedAverager (fix or create an issue)
• TimedStorage: add .pop() method
• MPFuture: make sure await asyncio.create_task(mpfuture) works without extra wrappers
• make alias DHTID.from(x) = DHTID.generate(source=x)? (@justheuristic )
• Use PeerID (that supports XOR distance itself) instead of DHTID
• Move test_dht_protocol() and test_empty_table() from test_dht_node.py to a new file test_dht_protocol.py (these tests use only DHTProtocol, not DHTNode)
• rename PeerID -> Endpoint, use bytes for PeerIDs in protobufs (instead of string)

[justheuristic]

Note: we decided not to implement TimedStorage.pop: neither of its 5 use cases would actually benefit from this method: all of them make deletion conditional, i.e. if item = s.pop(); if time_is_right: del s[key].