We already delete SMS which have been sent successfully. However, there
are plans to accept SMS for any subscriber in order to fix the problem
described in https://osmocom.org/issues/2354 ("SMSC: Store&Forward not
working for subscribed but unregistered MS").
This means we may end up storing SMS which never get sent, e.g. because
the B subscriber doesn't actually exist. This could lead to a higher
degree of SMS database growth over time, and therefore we need a way
to keep database size under control.
As a first step, introduce a DB function which removes an expired SMS,
and add a VTY command which removes all expired SMS from the DB.
Later commits will build upon this to remove expired SMS automatically.
The SMS expiry time period is currently hard-coded to 2 weeks.
We could make this configurable in the future if desired.
Change-Id: Icd6093b7b5d8db84b19a0aa47c68182566113ee2
Related: OS#2354
87cba1f105