artifactcache: Move pull logic into CASRemote

Seperates the pull logic into a remote/local API, so that artifact cache
iterates over blob digests checks whether it has them, and then requests them
if not. The request command allows batching of blobs where appropriate.

Tests have been updated to ensure the correct tmpdir is set up in process
wrappers, else invalid cross link errors happen in the CI.

Part of #802