'innodb_page_cleaners' option is added to perform flushing dirty pages
and keeping free pages for each buffer pool instance in parallel.
Increasing the value of the options increases scalability of the activity.

'innodb_page_cleaners'
Variable Scope   | Global
Dynamic Variable | No
Type             | numeric
Default          | 1
Range            | 1 .. 64

This patch was originally written by Inaam Rana,
and refactored by Yasufumi Kinoshita.

rb#3004 Approved by Kevin Lewis and Marko Makela