During a recent pcap trace, it was spotted that subscriber coming from
SGs had a use count with 16 "SGs" items, and later it incremented to 17.
Further investigation shows that the related use_count item was never
decreased, meaning every time an SGs-LU was sent by the MME, the item
was incremented further and never decremented.
Let's rename the item to be referenced while in LU, and then decremented
when LU is done. At that time, either the LU was accepted and the
subscriber object has a use_count item "attached", or it was rejected
and we already sent the reject messages, so we are fine deleting it if
needed.
Related: SYS#5337
Change-Id: I22c386f02ffa57428f700b003cc2cf23133598d0