Automatically resume interrupted downloads

[coryan]

Downloads can fail to start, we need a retry policy to restart the download if the connection is not established.

Downloads can fail after they start: we need a different policy to resume such connections. The resume policy may resume on more errors, and maybe does not backoff.

To safely resume a download we need to capture the generation of the object, this is available as a header when the object download starts. Then we should resume using the same generation, to avoid RAW hazards.