Number of messages unrecoverable due to NAK generation expiration (LBT-RM, LBT-RU)

- n (Int32)
- Statistics set index

Number of datagrams that were unrecovered
(LBM_MSG_UNRECOVERABLE_LOSS message was delivered) because a
retransmission was not successfully received within the NAK
generation interval.

This is controlled by the transport's
nak_generation_interval, with a default of 10000ms. This field
also includes messages declared unrecoverable due to a burst loss
event (LBM_MSG_UNRECOVERABLE_LOSS_BURST).
This method returns a value from the nth set of statistics
encapsulated within this object.