SS/USSD: make NCSS session timeout configurable

It may happen that either the MS or an ESME would become
unresponsive, e.g. due to a bug, or a dropped message.
This is why we have SS session timeout, that prevents
keeping 'stalled' sessions forever.

Let's introduce a VTY option, which can be used to configure
this timer (by default it's set to 30 seconds):

hlr
  ...
  ! Use 0 to disable this timer
  ncss-guard-timeout 30

Change-Id: I971fc2cee6fd46d4d5d6dac6c634e0b22fff183d
Related: OS#3717
diff --git a/tests/test_nodes.vty b/tests/test_nodes.vty
index e1f57a9..63d3da0 100644
--- a/tests/test_nodes.vty
+++ b/tests/test_nodes.vty
@@ -78,6 +78,7 @@
   no ussd route prefix PREFIX
   ussd default-route external EUSE
   no ussd default-route
+  ncss-guard-timeout <0-255>
 
 OsmoHLR(config-hlr)# gsup
 OsmoHLR(config-hlr-gsup)# list