Use direct ingestion API in the receiver worker process (GW)

Publishing files through a repository gateway involves placing the files in objects packs which are sent to the gateway.

The gateway application streams the object packs to the cvmfs_receiver worker process, who extracts each file from the pack, saves them to a temporary file on disk and uploads this file to the repository storage.

There could be a considerable performance gain if the objects packs are treated as an ingestion source and the files are directly ingested into the repository without any temporary files on disk.