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.
SQLite has this crappy type system where you can put a string into
a boolean or integer column, which of course will break once you try to
read it as bool..
Also, add an index by IMSI to the SUBSCRIBER table.