Deliver implementation guidance around overlapping subscriptions

Continue to allow the choice of behaviour, as today, but qualify it with additional text that describes its relation to subscription identification (subject of JIRA https://issues.oasis-open.org/browse/MQTT-253 ) Existing text in 3.5.3 “When Clients make subscriptions with Topic Filters that include wildcards, it is possible for a Client’s subscriptions to overlap so that a published message might match multiple filters. In this case the Server MUST deliver the message to the Client respecting the maximum QoS of all the matching subscriptions [MQTT-3.3.5-1] . In addition, the Server MAY deliver further copies of the message, one for each additional matching subscription and respecting the subscription’s QoS in each case.” Add the following, immediately after the above... "If the client provided subscription identifiers for any of the subscriptions involved, those identifiers MUST be returned in these published messages. If the server sends a single copy of the message it MUST include all the relevant identifiers in that PUBLISH packet. If the server sends multiple PUBLISH packets it MUST mark each one with the identifier of the corresponding subscription, if one was provided by the subscriber. "