* Be consistent in use of GetOrCreateAsync overload in exists and retrieval.
Ensure nullability of ContentCacheNode is consistent in exists and retrieval.
* Applied suggestion from code review.
* Move seeding to Umbraco application starting rather than started, ensuring an initial request is served.
* Tighten up hybrid cache exists check with locking around check and remove, and use of cancellation token.
* Optimize document and media seeding by looking up from database in batches.
* Ensure null values aren't stored in the cache when checking existance.
* Fixed failing integration tests.
* Resolved issue with not writing to the L1 cache on an L2 hit.
* Tidied up and populated XML header comments.
* Address issue raised in code review.
