libmsc: Track and update the location update expiry

Set the subscriber expiry timeout to twice the duration of the location
update period and provide functions subscr_expire() and
db_subscriber_expire() to mark subscribers offline that have missed two
location update periods.

This patch increases the DB revision to 3, so the hlr will be
incompatible with prior versions.

We should allow 0 for T3212 as well to disable the location update
period. In that case we will need a way to indicate that in the
database.
diff --git a/openbsc/include/openbsc/gsm_data.h b/openbsc/include/openbsc/gsm_data.h
index ea9f601..feb692f 100644
--- a/openbsc/include/openbsc/gsm_data.h
+++ b/openbsc/include/openbsc/gsm_data.h
@@ -247,6 +247,9 @@
 	int T3122;
 	int T3141;
 
+	/* timer to expire old location updates */
+	struct osmo_timer_list subscr_expire_timer;
+
 	/* Radio Resource Location Protocol (TS 04.31) */
 	struct {
 		enum rrlp_mode mode;