Fix when containerd restarted, event handler may exit
Fix when containerd restarted, event handler may exit

Description:
  Kill docker-containerd continuously, and use `kill -SIGUSR1
<dockerpid>` to check docker callstacks. And we will find that event
handler: `startEventsMonitor` or `handleEventStream` will exit.

This will only happen when system is busy, containerd need more time to
startup, and the monitor gorotine maybe exit.

Signed-off-by: Wentao Zhang <1f81ed848dd279299c89d57647e67fb988aa75e2@huawei.com>

moypray/docker,moypray/docker,moypray/docker