Allow deletion of QoS1/2 messages in rare circumstances

3.1.2.3.1 Clean Session, Page 19, Line: 559 Replace para starting "State could be lost" with:- In exceptional circumstances, the client or server MAY delete session state, or QoS 1 and 2 messages, at any time if required to do so because of:- resource constraints; limitations of the storage mechanism used; partial or complete corruption of the storage mechanism; administrative action, or on becoming aware of external knowledge that a message or messages can no longer be delivered to any current or future client. Furthermore, a client or server MAY delete QoS 0 messages at any time for the same conditions as bulleted in the preceding list. This could result in the loss or duplication of messages regardless of the QoS used. Non-normative comment The exceptional circumstances exist to cover that which can not be overcome. Implementations should make all possible efforts to preserve QoS 1 and 2 messages such that in practice their delivery guarantees are met during normal operation. The intent of this exemption is to accommodate scenarios such as:- the wear-levelling failures of solid-state storage the exhaustion of storage quota restrictions to prevent Denial of Service attacks unavoidable disk failure clients using the service being retired from service so their messages are no longer of value hard system failures, eg due to data centre power failure For QoS 0 messages, the constraint is there to make it clear that QoS messages are ephemeral with no guarantees of delivery or even visibility within the client or server. As a consequence of these constraints, implementors should be aware that loss or duplication of messages is possible regardless of the QoS used and should program their systems to be idempotent to this possibility.