Don't rely on kill return value for fallback path

Don't rely on kill return value for fallback path

Summary:
The `if kill(pid, SIGUSR2) != 0 then kill(pid, SIGKILL)` doesn't make sense. From the `man 2 kill` page:

```
ERRORS
       EINVAL An invalid signal was specified.

       EPERM  The process does not have permission to send the signal to any of the target processes.

       ESRCH  The process or process group does not exist.  Note that an existing process might be a zombie, a process that has terminated execution, but has not yet been wait(2)ed for.
```

Kill only **sends** the signal and makes sure it **arrives**. It makes no guarantee about whether the receiving process **properly interpreted the received signal!**

This explains why my `monitor_kill_again_fix` isn't as effective.

 ---

Fix: just start sending SIGKILLs after some time.

Reviewed By: ljw1004

Differential Revision: D30372367

fbshipit-source-id: cf0f133088628620821b71f49ca796210cf1c1e1