docs: def client(self, name=None):
          """Initialize a backend's client with given name or default."""
          name = name or self.default
          if not name:
              return NullClient(self, None, None)
          params = self.backends_hash[name]
          ccls = self.backends_schemas.get(params.scheme, TCPClient)
          return (yield from ccls(self, params.hostname, params.port).connect())