Be more liberal when the similar content of the post to be saved turns out to be the same post

 - instead of setting a random number, we actually store the post id in redis now
 - and when checking for matching recent post and it is the same, we allow it still

Fixes the issue that we prevent a user from reverting to a previous state within the unique_posts_min time window.
Merge pull request #2072 from ligthyear/fix-edit-body-too-similar-issue

Be more liberal when users wants to change their own body to a previous version