vfs: Introduce a new cache warmer. - #504
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* It's too brittle, and breaks on bigger hierarchies.
* This one uses GetBlobs, and is deterministic in its order as opposed to the Prefetcher.
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This gets rid of the existing VFS cache prefetcher, because it falls down on bigger hierarchies, and switch to a deterministic VFS cache warmer.
The design is quite simple, we just introduce a new step in the backup pipeline that groups parents dir, and warms the VFS cache. This warming happens using GetBlobs and so is super efficient (hence the need to group!).
This turns a second backup (no changes) on the linux kernel on an AWS store from 3min to 18s.
ps: Still a draft because I'm not a huge fan of the PR history and I need some fixups. But it can already be looked at and used!