Efficient and reliable download via the Vault

This is a meta-task to track the development of a next generation, efficient and reliable dowload mechanism for revisions via the Vault.

In order to produce efficiently a git bundle/git bare output, one should use in priority the swh-graph service to extract all the objects, and fall back to the old less efficient approach if swh-graph does not yet include the requested roots.

Vault related tasks can be found looking at the Vault tag