Adding ability to override the CacheItemPolicy within the creation delegate for a new cache item.

Fix to ensuring locking on first request.

Removing normalisation of case around background thread locking.

Fixes to locking.

Adding support for cancellation tokens. If update fails the update can be cancelled and force continued used of stale cache.

If no cache items exist and the request if cancelled, a null value will be returned to caller.

API tweaks to better support thread safety.

Initial release.