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
6 files changed