Bug False positive "Replier can only add posts at the end of a thread" from non-monotomic time()

[Xon]

php's time() function is non-monotomic. This means it can technically go backwards, noticeable on machines which use a timesync service on a very busy forum

Consider the case;

• UserA starts replying
  • finalSetup - Clock N, Post.post_date = N
• Clock progress to N+1
• Another users posts, updating the xf_threadrecord
  • xf_thread.last_post_date = N+1
• Timsync causes the clock to be set to Clock N
• UserA's request reloads the last post...

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