As CASRemote is used in other places such as SandboxRemote it makes sense for
there to be a tmpdir it uses. This dir is passed to CASRemote when initialized.

This is currently in the artifactdir, but should be moved when artifactdir and
builddir options are deprecated in favour of one top level directory containing
the two.

Part of #802

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. Additional asserts
have been added to check that the temporary directories are cleared by the end
of a pull.

Part of #802

Similar to the pull methods, this implements a yield_directory_digests methods
that iterates over blobs in the local CAS, with the upload_blob sending blobs
to a remote and batching them where appropriate.

Part of #802