Move net/connect.py to neubot_asyncio/_connect.py

Since asyncio's create_connection() loops through all the ainfos
returned by getaddrinfo(), the connect() semantic of Neubot can be
implemented by just supplying to create_connection() the correct
address families. Depending on prefer_ipv6, AF_INET first and then
AF_INET6, or the other way round.

So, we don't need much new connect() logic in net/, rather we
need to separate the logic from prefer_ipv6 and to move the code
within the neubot_asyncio directory.