Large message's copy may be interfered by other threads

In LargeMessageImpl.copy(long) it need to open the underlying file in order to read and copy bytes into the new copied message. However there is a chance that another thread can come in and close the file in the middle, making the copy failed with "channel is null" error.

This is happening in cases where a large message is sent to a jms topic (multicast address). During delivery it to multiple subscribers, some consumer is doing delivery and closed the underlying file after. Some other consumer is rolling back the messages and eventually move it to DLQ (which will call the above copy method). So there is a chance this bug being hit on.