message: io: reduce unknown error code log level
  ```
  2019-07-29T17:11:16.00568 [2019-07-29 17:11:16.005][26039][error][misc] [external/envoy/source/common/network/io_socket_error_impl.cc:29] Unknown error code 111 details Connection refused
  ```
  I saw the previous log line on a production box and I think this should
  be a debug log condition.
diff: @@ -26,7 +26,7 @@ Api::IoError::IoErrorCode IoSocketError::getErrorCode() const {
  case EADDRNOTAVAIL:
  return IoErrorCode::AddressNotAvailable;
  default:
  - ENVOY_LOG_MISC(error, "Unknown error code {} details {}", errno_, ::strerror(errno_));
  + ENVOY_LOG_MISC(debug, "Unknown error code {} details {}", errno_, ::strerror(errno_));
  return IoErrorCode::UnknownError;
  }
  }
  