Uses the same method MEE6 uses for reliable cost effective scanning of all reddit posts.
Works by by manually checking 100 ids per couple of seconds via the /api/info.json?id=t3_id1,t3_id2 etc route.
It then updates the cursor to the highest returned post id, continuing from that next call.
I don't believe this method has any faults at this moment, it seems to be more than enough even at a 5 second interval, unlike the polling of /all/new this does not appear to have any limitations on old posts and also shows absolutely all posts.
reddit_last_post_id id of last post processed
global_subreddit_watch:sub
Type: hash
Key: {{guild}}:{{watchid}}
Value: SubredditWatchItem
guild_subreddit_watch:guildid
Type: hash
Key: {{watchid}}
Value: SubredditWatchItem
watchid is a per guild increasing id for items