Add inflight message size to SubscriptionStatistics

It would be useful to know the total message size of messages that are inflight (besides just the count) on a Subscription for monitoring and metrics purposes. This value can be added to SubscriptionStatistics and kept in up to date so it can be displayed without blocking.

A future enhancement when AMQ-5748 is done could be to add a pending message statistics. AMQ-5748 is required first to be able to recover message sizes across a broker restart.