WL#9625: Enable pfs memory instrumentation by default

Affects: Server-8.0   —   Status: Complete   —   Priority: Medium

In MySQL 8.0 we should enable pfs memory instrumentation in the default 
configuration of the server.

This is important because the accounting is incorrect if instrumentation is 
enabled after server start (i.e. you could get a negative balance from missing an 
allocation, but catching a free).

User Documentation
==================

* https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-2.html
* https://dev.mysql.com/doc/refman/8.0/en/memory-summary-tables.html
FR1) The compiled default configuration of the MySQL Server is changed to:
performance-schema-instrument='memory/%=COUNTED'