WL#9630: Change innodb_max_dirty_pages_pct_lwm default to 10

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

In MySQL 8.0, we would like to change the default of
innodb_max_dirty_pages_pct_lwm to 10.

This is the default we are currently using in OPC MySQL, and should result in
more stable throughput.

We should also expect the default configuration to have larger buffer pools and
thus benefit more from pre-flushing enabled.

** Upgrade Story **

With innodb_max_dirty_pages_pct_lwm=10, InnoDB will increase its flushing
activity when >10% of the buffer pool contains modified ('dirty') pages.  The
motivation for this default change is to trade off peak throughput slightly, in
exchange for more consistent performance.  We do not expect the majority of
users to see impact from this change, but symptomatically query throughput may
be reduced after a number of sustained modifications.  Users who wish to revert
to the previous behavior can set innodb_max_dirty_pages_pct_lwm=0.  The value of
zero disables the increased flushing heuristic.
FR-1) The compiled default value of innodb_max_dirty_pages_pct_lwm changed from 0
to 10.

NFR-1) Any changes required to MTR tests.