Download encrypted chunks early

[Deleplace]

IF all of the following conditions are met :

• webapp supports partial download,
• webapp supports encrypted download decypher,
• there aren't many web clients open at this time in this region (say, less than 5),
• mobile app has already encrypted and uploaded partial or full data, but hasn't scanned QR-code yet,

THEN all the nearby web clients may start downloading the encrypted partial data already uploaded. It would be decrypted only by 1 web client after the QR-code has been scanned and the mobile app has provided the decryption key for a single target.

[Deleplace]

Also, we should restrict to targets in same country, and prioritize those having small estimated distance from mobile source.

[Deleplace]

See also #60 [Streaming]

[Deleplace]

The current issue is about partial resource data, and geo-based heuristic.

See #514, #515 about full resource data, and history-based heuristic.