Returns true if this executor is in the process of terminating
after {@link #shutdown} or {@link #shutdownNow} but has not
completely terminated. This method may be useful for
debugging. A return of {@code true} reported a sufficient
period after shutdown may indicate that submitted tasks have
ignored or suppressed interruption, causing this executor not
to properly terminate.
@return {@code true} if terminating but not yet terminated
Returns true if this executor is in the process of terminating after {@link #shutdown} or {@link #shutdownNow} but has not completely terminated. This method may be useful for debugging. A return of {@code true} reported a sufficient period after shutdown may indicate that submitted tasks have ignored or suppressed interruption, causing this executor not to properly terminate.
@return {@code true} if terminating but not yet terminated