WAIT_NESTED_CONTAINER sometimes returns 404

WAIT_NESTED_CONTAINER sometimes returns 404s even though the nested container has already exited and the parent task/executor is still running.

This happens when an agent uses more than one containerizer (e.g.,
docker,mesos, WAIT_NESTED_CONTAINER and the exit status of the nested container has already been checkpointed.

The root cause of this is a bug in the ComposingContainerizer in the following lines: https://github.com/apache/mesos/blob/1c7ffbeb505b3f5ab759202195f0b946a20cb803/src/slave/containerizer/composing.cpp#L620-L628