Number of datagrams unrecovered due to transmission window advance (LBT-RM only)

- n (Int32)
- Statistics set index

Number of datagrams unrecovered (LBM_MSG_UNRECOVERABLE_LOSS delivered to
receiver application) due to transmission window advance.

This means that the message was no longer in the source-side transmission
window and therefore not retransmitted. The window size is set by transport
configuration option lbtrm_transmission_window_size (default 24MB).
This method returns a value from the nth set of statistics
encapsulated within this object.