LSN age factor tuning for adaptive flushing in 5.6

With the tuned page cleaner heuristics [1], adaptive flushing becomes too aggressive and maintains a consistently lower checkpoint age than a similarly-configured stock server. This results in a performance loss due to reduced write combining.

One way to address this is to reduce LSN age factor at low LSN ages and increase it at high LSN ages with a different option.

Introduce new system variable innodb_cleaner_lsn_age_factor=LEGACY|HIGH_CHECKPOINT, default HIGH_CHECKPOINT, to control the formula choice.

[1] https://blueprints.launchpad.net/percona-server/+spec/cleaner-tuning-5.6