* 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.
(cherry picked from commit 81a8a0c191)
68d1b9481a