osmo-sgsn: ping GGSN periodically and check for restart counter
Before this commit, echo req/rsp logic was implemented in libgtp but
never used in osmo-sgsn.
This commit adds a timer which periodically sends a GTP ECHO Request to
every GGSN if there's at least one pdpd context associated with it. This
way by checking the restart counter in the ECHO Reply it can be known if
the GGSN was restarted. In this case, logic already present in osmo-sgsn
will terminate all pdp contexts associated with that GGSN.
Change-Id: I9d714726785407859f26bbef052cd0efc28e8dae