Changed termination logic of the default executor.
Changed termination logic of the default executor.

Previously, the default executor terminated itself after all containers
had terminated. This could lead to termination of the executor before
processing of a terminal status update by the agent. In order
to mitigate this issue, the executor slept for one second to give a
chance to send all status updates and receive all status update
acknowledgements before terminating itself. This might have led to
various race conditions in some circumstances (e.g., on a slow host).
This patch terminates the default executor if all status updates have
been acknowledged by the agent and no running containers left.
Also, this patch increases the timeout from one second to one minute
for fail-safety.

Review: https://reviews.apache.org/r/72029

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