Adds a purge threshold setting. This setting allows the user to
control at what filled percentage the purge activities are run.
This replaces the default, hard coded 95% point.
The purge threshold defaults at the original 95%. I set a minimum
of 20 and max of 99 because those values felt sensible but am open
to changing those based on feedback.
Note: The purge threshold is still active when the keep option is
set. I added a note for this but this still presents some risk
where users who change this while in Keep mode could have their
services shut down earlier than they expect.
Christopher Leinbach