https://issues.apache.org/jira/browse/AMQ-5290

Add some utility methods that are useful for restoring past
subscriptions.
https://issues.apache.org/jira/browse/AMQ-5290

Reduce memory overhead of the MQTT Protocol converter by not storing the
UTF8Buffer instances and instead simply store the needed String values.
We always access the String value anyway so all of the UTF8Buffer object
we store carry the overhead of both marshalled UTF-8 bytes and an
unmarshalled String object.