send GSUP PurgeMS Request (CN-Domain = CS) to the HLR upon IMSI-Detach
OsmoSGSN sends a GSUP PurgeMS Request to the HLR when a subscriber detaches (with CN-Domain = PS).
OsmoMSC does not send such a message.
But: this could instead be a message that should be sent to a previous MSC/SGSN upon an attach on a new MSC/SGSN.
Make sure beyond doubt that sending PurgeMS is the proper thing to do for IMSI Detach;
If not, reject this issue and add an OsmoSGSN one to fix OsmoSGSN's PurgeMS behavior.
If it is correct to send PurgeMS, make OsmoMSC also send this message with CN-Domain = CS.
Figure out whether to send a PurgeMS only for explicit IMSI Detach, or also for implicit detach due to periodic LU timeout.
For completeness' sake also check/match up OsmoSGSN's behavior for implicit detach.