LP #1532747: Page cleaner worker threads are not instrumented for performance schema

**Reported in Launchpad by Laurynas Biveinis last update 12-01-2016 09:54:33

Copy of https://bugs.mysql.com/bug.php?id=79894:

[8 Jan 13:56] Laurynas Biveinis
Description:
Credit shared with Alexey Stroganov.

Page cleaner worker threads do not call pfs_register_thread.

This means:
1) (if I recall correctly how PFS thread instrumentation works) no events by these threads are actually accounted.
2) they are invisible in PERFORMANCE_SCHEMA.THREADS table.

If 1) is correct, then any results so far on 5.7 to gain insights into MT page cleaner performance by PFS are basically invalid. To verify 2), start server in default configuration, and query P_S.THREADS. You will see one cleaner thread there only. The three worker threads there are purge. The cleaner worker threads are not there.

How to repeat:
See above, check buf0flu.cc for buf_flush_page_cleaner_worker NOT calling pfs_register_thread.

Suggested fix:
Add a new thread key, register cleaner worker threads with PFS.