Implement a proper subscriber connection mapping. Create a new
subscr_conn when one of the stored BSCs sends a conn.ind and use that
subscr_conn for all related connection-oriented messages with the same
RAN-side conn_id to send messages back and forth between BSC and MSC.
Add subscr_conn_get_next_id based on OsmoBSC's
bsc_sccp_inst_next_conn_id.
With this patch, it's possible to do a successful Location Updating
procedure between MS2 and MSC in the following network:
MS1 --- BTS1 --- BSC1 --.
BSCNAT --- MSC
MS2 --- BTS2 --- BSC2 --'
Related: SYS#5560
Change-Id: I1556aa665fbb0a97507f98794e74820731fa6935
