Update db.py

[clarksun]

trafficstars

if dbw is a batch server or score log process server, I mean this db process do not consume spider log topic, After add no-incoming flag to db launch command, group dbw-spider-log always log 'Heartbeat session expired. so I add this condition to prevent init spider_log.consumer

[sibiryakov]

Hi, have you tested it @clarksun ? Is DBW with --no-incoming option working?

[clarksun]

@sibiryakov this is our dbw. https://gist.github.com/clarksun/f651c2200164ce9999428d858e67d81b

batch server and scoring server both add --no-incoming, and works well.

[Prometheus3375]

Seems like this PR is outdated, so it can be closed.