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 (

   firebolt/executor/executor.go

   Line 264 in 223d445

   time.Sleep(10 * time.Second)

   )
2. During this period of time, a shutdown message is received by app (

   firebolt/executor/executor.go

   Line 271 in 223d445

   func (e *Executor) Shutdown() (done chan struct{}) {

   )
3. Executor try to shutdown source, but old instance (

   firebolt/executor/executor.go

   Line 274 in 223d445

   err := e.source.Shutdown()

   )
4. Executor continues to execute an infinite loop (

   firebolt/executor/executor.go

   Line 250 in 223d445

   for {

   )
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!