neubot_asyncio: disband _utils.py

Move the _utils.py code part in _eventloop.py and part
in transports.py.
transports.py: use future for _TCPConnector

On second thought, a future is better than a custom event emitter,
because the future correctly routes unexpected errors.
neubot_asyncio: make _Server more similar to _TCPConnector
transports.py: simplify bind()

There is no need to sort the addrinfos, because now we use the
trick (copied from the real asyncio) that we configure Linux to
behave like a dual stack TCP/IP system.