Fixed concurrent wait within the ExternalContainerizer.

The ECP must not receive multiple, concurrent wait-invocations on the
same ContainerID.

This fix checks if a wait-invocation is already active by checking if
the wait-PID is setup (isSome). If that is the case, the EC simply
returns the future of the termination without additionally invoking
the ECP.

Review: https://reviews.apache.org/r/21328