userstat per query, session (currently only global)

Statistics should be tracked at 3 different levels, per query, per session, and globally.

Currently, we only track this globally.

Background: A patch was submitted to 5.2 in 2011 and 2012 that did this. Details are now difficult to find and involve the wayback machine. Here is some information: https://planet.mysql.com/entry/?id=28194

Discussion: https://www.mail-archive.com/search?l=maria-developers@lists.launchpad.net&q=subject:%22%5C%5BMaria%5C-developers%5C%5D+Changes+to+table_statistics+for+mariadb+5.2%22&o=newest&f=1

I am unable to locate the patch that was submitted itself, but it was not put into the codebase.

Customer continued using patched 5.2 until now and cannot upgrade without this feature.