Check Will publish permission on connect

[popduke]

Motivation

A Will message is declared as part of the CONNECT packet, so its publish permission should be determined before the session is established.

Previously, the permission was checked only when the Will was about to be published. That could happen long after CONNECT and make the result depend on permission state at disconnect time.

Changes

• Check the Will topic's publish permission while processing CONNECT.
• Reject CONNECT with the corresponding MQTT 3 or MQTT 5 reason code when the permission is denied or cannot be checked.
• Remove the permission check from the Will publication path.
• Keep normal PUBLISH permission checks unchanged.

Testing

Added MQTT 3 and MQTT 5 coverage for:

• denied Will publish permission;
• permission provider failures and invalid results;
• immediate and delayed Will messages;
• permission checks occurring before session creation;
• Will publication without another permission check.

[Gujiawei-Edinburgh]

It looks good to me.