This adds directory objects to the local repository before downloading
files in the directory. However, artifact references are still stored
only after downloading the complete directory and thus, there won't be
dangling references. This will anyway be required for partial download
support.

gRPC can handle 1 MiB payloads. Increase size limit from 64 KiB to speed
up uploads.`

Use 1 MiB as payload size limit on the server side for both individual
downloads and batch uploads.

This uses BatchReadBlobs instead of individual blob download to speed up
artifact pulling, if the server supports it.

Fixes #554.

_artifactcache/cascache.py: Use BatchReadBlobs

Closes #554

See merge request !813