Slave should make sure that the containerizer::launch returned Future is ready

Currently the slave is not awaiting the Future<Nothing> returned by Containerizer::Launch before sending out more command events.

Is there a reason for this behavior?

This issue becomes apparent only when having a launch-command-implementations that is relatively "expensive".

So what I can see here is the following chain of events along a vertical time axis:

Launch
   |
   |  Wait
   |    |
   |    | Update 
   |    |    |
   ----------Launch Future<Nothing> became ready

What I would like to see is:

Launch
   |
   |
   |
   |
   |
   ----------Launch Future<Nothing> became ready
      Wait
        |
        | Update 
        |    |

As we are currently pushing the former behavior into the implementation of the containerizer, things quickly get rather complicated on that side. Hence I would like to understand if that is something we really want / need or if we might want to fix this within the slave in a longer run.

So far, I have only observed this to be a challenge for Launch, but other events might just as well be worth a thought on enforced chaining instead of concurrent invocations.