Returns {@code true} if all tasks have completed following shut down.
Note that {@code isTerminated} is never {@code true} unless
either {@code shutdown} or {@code shutdownNow} was called first.
@return {@code true} if all tasks have completed following shut down
Returns {@code true} if all tasks have completed following shut down. Note that {@code isTerminated} is never {@code true} unless either {@code shutdown} or {@code shutdownNow} was called first.
@return {@code true} if all tasks have completed following shut down