KahaDB durable subscription stats can go negative on duplicate acks

AMQ-6375 added support for non blocking durable subscription statistics to KahaDB to track pending message count and message size metrics. By default this is disabled and can be enabled with the enableSubscriptionStatistics flag on KahaDB.

On rare occasions I have noticed that sometimes the metrics may got negative or be slightly off. I found an area where it's possible if a duplicate ack comes in then in some scenarios the metrics may be deprecated twice. This issue only applies to the subscription stats and not the store statistics for the overall destination as those stats would only get decremented once when the message is removed from the order index.