[A-MQ6, producer flow control, destination memory usage] destination memory usage incorrect after sendtimeout due to PFC.

If sendtimeout is exceeded in a transaction due to Producer Flow control blocking, and the transaction is rolledback before the Producer flow control frees: the memory usage for the destination is not reduced correctly and will stay at an incorrect value.

sendtimeout is important for this use case, as it will use syncsend within a transaction.

Normally a transaction rollback would decrement the memory usage but it seem the rollback does not call the appropriate decrement when the message is blocked by the PFC.