Convert DHT to libp2p backend

[borzunov]

Current status: finishing the docs and minor details, ready for review.

What I have tested:

• Unit tests work
• Benchmarks work
• In examples/albert, both run_training_monitor.py and run_trainer.py work. Training with 2 GPU peers + 1 training monitor performs successful averagings and gradually reduces the loss.

Plan:

• Make DHTProtocol work over P2P
• Make DHTNode work over P2P
• Make hivemind.dht.DHT work over P2P
• Convert test_dht_node.py
• Convert test_dht.py
• Convert other tests matching test_dht_*.py
• Convert tests not related to DHT directly
• Convert DHT usages outside tests
• Fix docs (initial_peers/listen_on/dht_port/port in *.md)
• Fix duplicated logging in run_training_monitor.py

Details:

• Fix AuthRPCWrapper usage with Role = Server
• Terminate all p2pd instances
• Remove all Unix socket connections (and not interfere with other processes)
• Handle SIGTERM in test_dht_node.py
• Mimick gRPC servicer (make ServicerBase and a P2P method to add it?)
• Other TODOs (if present)

Ideas for future PRs:

• Add test for use_ipfs=True
• Implement simplified multiaddr sharing
• Thoroughly check graceful shutdown
• Call drain() after all StreamWriter.writes
• Rewrite do_handle_stream(): send data in chunks, call drain(), don't concat a big string with its size

Related issues: #91 #149 #165 #185 #238