Using this procedure, the VLR/SGSN can set the cs/ps purged
flag for the subscriber. We might not even need to store this
persistent in the database according to spec, but let's do it anyway, at
least until it turns out to be a performance issue.
When responding to a SendAuthInfo.req, we need to differentiate
an error case caused by an unknown IMSI, or an error caused by
an error regarding accessing the database or data integrity.
We also introduce a 'gsup_router' which enables us to route
a transaction to a given VLR. It works based on the SERIAL attribute
communicated at time of the IPA multiplex setup as part of the CCM
sub-protocol.