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

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.

Also see https://github.com/ligthyear/discourse-plugin-checklist/issues/2 and https://meta.discourse.org/t/checklist-a-simple-markdownified-checkbox-plugin-with-inline-updating/13428/7?u=bhaelochon