DefaultNotificationCacheManager shouldn't rely on a document configuration source

As exposed in XWIKI-22954 right now DefaultNotificationCacheManager injects DefaultNotificationConfiguration which relies on DefaultConfigurationSource. That later uses 2 sources of configuration: the xwiki.properties and the document configuration source.

The problem is that DefaultNotificationCacheManager is initialized globally for the farm since the extension is installed at farm level, so it doesn't make sense that it reads value from a specific wiki document. Especially since the wiki where it reads the value depends on how it's initialized: e.g. an event triggered on a specific wiki might initialize it with wrong values.