Deleting a post does not remove it from unread lists of users

[julianlam]

Note: This issue could be exploited to "bump" posts in front of viewers without actually replying to a post.

Repro

1. Reply to a topic as account A
2. Delete said post
3. Account B will have that topic in their unread list, even though there was nothing new.

Gotchas

1. Take care not to remove the topic from the unread set/list/zset if it was already there before the new reply

[julianlam]

The tricky part of this is the way we currently handle unread state for a topic. It doesn't quite lend itself to a simple solution.

Best thing to do would be to refactor how we record the "last pid read" on a per-topic basis, which will allow us to resolve this issue.

[julianlam]

Unfeasible given how we handle posts per user at current.