Change recent sensor_packet 'used' field into 'tri_id

On second consideration, it made more sense to allow the field to
store more than Boolean values.  This allows the retrieval of
sensors used in a given triangulation, which will be used in
Replay Mode eventually
Implement getRecentPacketSel

PHP Function is used to make query string for all recent (within a
provided offset) packets that aren't attached to a triangulation