Allow double-deletes of rows if requested.

When issuing a DELETE stmt from Impala that performs a joint it is possible that the generated set of rows is not distinct, which can lead to double deletes of single rows. Right now, we iterate over the errors in the session and ignore Status::IsNotFound. In the future, it would be nice to simply pass a flag to KuduTable::NewDelete that does not propagate those errors back to the client.