Cancels any pending update for this cache entry. This should
only be called by the thread that is responsible for performing the update ie the thread that received the original {@link NeedsRefreshException}.
If a cache entry is not updated (via {@link #putInCache} and this method isnot called to let OSCache know the update will not be forthcoming, subsequent requests for this cache entry will either block indefinitely (if this is a new cache entry or cache.blocking=true), or forever get served stale content. Note however that there is no harm in cancelling an update on a key that either does not exist or is not currently being updated.
@param key The key for the cache entry in question.
@throws IllegalStateException if the cache entry isn't in the state UPDATE_IN_PROGRESS