Refactor requestUpdate() filtering.
Rename getFilterName to getUniqueName because it's used for more than just filter names.
Added a couple of utility methods.
- Refactor the scheduler so that it can be used in themes.
- Add state storage to the base update checker.
- Move the "allow metadata host" workaround to the base class.
- Rename cron hook from "check_plugin_updates-$slug" to "puc_cron_check_updates-$slug" and "tuc_cron_check_updates-$slug".