store the last SID frame of a given LCHAN so we can repeat it
store the last SID frame of a given LCHAN so we can repeat it

repeating the last SID frame is required during 20ms intervals where we
don't get a new SID update (SID RTP frames are valid up to 160ms)

osmocom/osmo-bts,shimaore/osmo-bts,telenoobie/osmo-bts,geosphere/osmo-bts,osmocom/osmo-bts,geosphere/osmo-bts,telenoobie/osmo-bts,tacooper/osmo-bts,shimaore/osmo-bts,osmocom/osmo-bts,geosphere/osmo-bts,tacooper/osmo-bts,shimaore/osmo-bts,telenoobie/osmo-bts,shimaore/osmo-bts,telenoobie/osmo-bts,geosphere/osmo-bts