Start a socket server, call back for each client connected.
The first parameter, `client_connected_cb`, takes two parameters:
client_reader, client_writer. client_reader is a StreamReader
object, while client_writer is a StreamWriter object. This
parameter can either be a plain callback function or a coroutine;
if it is a coroutine, it will be automatically converted into a
Task.
The rest of the arguments are all the usual arguments to
loop.create_server() except protocol_factory; most common are
positional host and port, with various optional keyword arguments
following. The return value is the same as loop.create_server().
Additional optional keyword argument is limit (to set the buffer
limit passed to the StreamReader).
The return value is the same as loop.create_server(), i.e. a
Server object which can be used to stop the service.