V15: Reimplement cache startup handler

[Zeegaan]

Notes

• Reimplements the NuCacheStartupHandler, now renamed to HybridCacheStartupNotificationHandler.
• This will rebuild the cache, if the NuCacheSettings.NuCacheSerializerType changes (on startup).

How to test

• Create some content in umbraco.
• Checkout that content, thus loading it into the cache.
• Stop umbraco, and change the setting in "Umbraco:CMS:Nucache"

"NuCache": {
        "NuCacheSerializerType": "JSON"
},

• Start umbraco and checkout the content, assert that it still works.