App can't be be normaly sopped if source wait to reload

[yellow-sky]

1. Source can't connect to kafka\other resource - executor wait 10 sedonds (https://github.com/digitalocean/firebolt/blob/223d445912e74ffe1d039584a89df03a222ae954/executor/executor.go#L264)
2. During this period of time, a shutdown message is received by app (https://github.com/digitalocean/firebolt/blob/223d445912e74ffe1d039584a89df03a222ae954/executor/executor.go#L271)
3. Executor try to shutdown source, but old instance (https://github.com/digitalocean/firebolt/blob/223d445912e74ffe1d039584a89df03a222ae954/executor/executor.go#L274)
4. Executor continues to execute an infinite loop (https://github.com/digitalocean/firebolt/blob/223d445912e74ffe1d039584a89df03a222ae954/executor/executor.go#L250)
5. New source instance doesn't know anything about studown signal
6. Go to step 4 => App does not respond to system shutdown signals

[jnadler]

Thanks for the bug report @yellow-sky - I can see how this is a problem if a firebolt app starts while kafka is unavailable. I'll work on fixing this when I can, or if you need it fixed quickly we'd be happy to consider a pull request!