update the cell id in the VLR when receiving BSSMAP HANDOVER PERFORMED from the BSC
osmo-msc receives the subscriber's current Cell ID in a Complete Layer 3 Information message.
When the BSC has performed an intra-BSC handover, the MSC is notified by a HANDOVER PERFORMED message, see 3GPP TS 48.008 126.96.36.199.
This contains the new Cell ID.
So far osmo-msc ignores the HANDOVER PERFORMED message.
Implement HANDOVER PERFORMED parsing and update both msc_a->via_cell and vsub->cgi with the new Cell ID.