Added first_ts and last_ts to cellloc structure.
Fill cellloc.first_ts and .last_ts like .ts.
Added index on cellloc.last_ts.
Changed index cellloc.cid: use last_ts rather than first_ts.
Don't use cellloc.ts anymore.  When retrieving, use last_ts
to get the most recent.
Removed cellloc.ts.