Caching invalidation is typically triggered by content distribution, after processing an `ADD` or `DELETE` distribution request. When distributing referenced content, invalidation must also include on the referencing resources. To support this use case, we'll add support for a new type of distribution request that invalidates the caches without importing content.
To support this, the org.apache.sling.distribution.DistributionRequestType API will be extended with
The PackageMessage.ReqType will be extended with the new INVALIDATION type.
The DistributionPublisher will handle the invalidation distribution requests by sending a package message with the INVALIDATION type and no payload (like the DELETE package messages).
The DistributionSubscriber will process invalidation messages by raising imported events and invoking the import post processors.