gprs-ns/stats: When the NSVCI is updated, update the stats counter

The NS object is created with an unknown identity and only after
the reset procedure has progressed (completed?) we know the real
ID for this peer. Before nobody has looked at the idx values (this
could have been seen with the CTRL interface) but with statsd the
wrong NSVCI becomes obvious.

Add routines to update the idx and I don't know if the change of
idx is causing any issues but we will find that out soon.
3 files changed