Motivation:
Subscription ID property of the PUBLISH message may be repeated multiple times, which wasn't taken into account when developing `MqttProperties` API.
Modification:
Store Subscription ID properties separately from others - in `MqttProperties.subscriptionIds`.
Add `MqttProperties.getProperties` method to retrieve properties that may be repeated.
Change internal representation of User Properties for uniformity with Subscription ID - now they're stored in `MqttProperties.userProperties` rather than the common hash map.
Result:
Multiple Subscription ID properties can be set or retrieved.
30e9b6846d