[AMQP 1.0] annotations added by broker use string keys rather than symbols, mutates existing symbol keys

Noticed while trying out QPID-5285, the broker adds an illegal string typed annotation key for the added timestamp.

Worse, the addition also causes the broker to mutate incoming symbol-keyed annotations to also use illegal string keys as well. As a result, even if a recipient could ignore the wayward type (which many cant) for any added annotations it wasn't specifically aware of, it would still find any annotations it was aware of to be broken.