Improve chunk seeding with gateway

[CDDelta]

To ensure that chunk data is seeded onto the network reliably and not lost in the case of node failure, the gateway should send a single /chunk request to multiple peer nodes.

I mentioned this in #8 but I wasn't too clear on it.

On a smaller note, sending a POST to https://gateway.amplify.host/chunk yields a Request type not found response so chunk seeding actually isn't possible atm. :)