Improve the performance of ParticipantAdder.

By making a wholesale query to fetch all existing Tf2rUsers with
steam_ids matching the participant list, we avoid making a gajillion
exists? queries.

The new convert_to_tf2r_users method will simply use an existing id (as
sought in the existing Tf2rUsers hash). If an existing Tf2rUser is not
found, a new one is created, as before.