Always purge non-shared stores on cache startup

purgeOnStartup is unsafe with shared stores, but it must be enabled for non-shared (private) stores in order to avoid resurrecting removed entries.

We can enhance the graceful cluster/cache shutdown operations to save a "disable purge" flag in the persisted state, and purge on cache startup the non-shared stores of any cache missing the "disable purge" flag. This will remove the need to ever disable purgeOnStartup.

Then we can ignore the purgeOnStartup setting and only log a warning that it will be ignored if the user enabled it for shared stores or disabled it for non-shared stores.