Reintroduce the PFS query sample

[svaroqui]

This feature was disable because it produce temp table on disk the way the data was fetch
I propose to have a slow background threads using handler https://mariadb.com/kb/en/handler-commands/

This feature was disable because it introduce security issues with store procedure using explain
https://www.youtube.com/watch?v=XBHlAwn-WbQ

/* COALESCE((SELECT B.SQL_TEXT FROM performance_schema.events_statements_history_long B WHERE
A.DIGEST = B.DIGEST LIMIT 1 ),'') as query, */ was replace per empty string to avoid full join

This feature was here to enable explain on the heavy query
Take care never take explain on unsafe query for security see Alexandre Rubin post on security