Resumable downloads.

[Narsil]

Notes on the strategy here.

Sync:

• Relatively straigforward, we're not multiplexing, we write in order, therefore we can simply pick up whereever we left off.

Tokio:
Since we're multiplexing, writes are not happening in order. We could:

• Store every written interval and skip those, but that involves creating somewhat complex datastructures.
• Instead, since writes are mostly ordered, we simply commit whatever was fully written as a single u64 at the very end of the partial file. When resuming, we read that single int, and resume from there. Anything fishy and we drop the resumability.

This feels simpler that the more complex version, seems to work quite well in practice (only tested on stable networks right now).
The probability for storing invalid files seems low (since commit happens at a single location both in code and in file).