reporter_kbpki: add a shutdown channel for goroutine sends
reporter_kbpki: add a shutdown channel for goroutine sends

Suggested by jzila.

Issue: #17118

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client