Launchpad ingestion to archive endpoint

Wire up launchpad ingestion to charmstore filling out new api/models.
There's no particular guarantee that ArchiveTo is deterministic (some other process might be traversing the fs and reading files, changing the file access times, for example). I still think it's probably better to archive to a temporary file (calculating the hash at the same time), then read from that. Then no need for a pipe (or Counter), although it will use more disk space.