Merge data from multiple peers

[feross]

This is a minor optimization.

Merge metadata from multiple peers to return metadata more quickly.

Need to be careful. If one peer consistently sends bad data, you'll get stuck retrying for a long time and have no idea which peer sent the bad data.

Here's how it will work: Assume peers are all good. Merge metadata from all peers together, but also keep metadata per-peer (like we're doing now). Merged metadata should finish fastest. If it matches sha1 hash, great, we're done! If not then just take metadata from first peer (like we're doing now).

[feross]

I have no plans to implement this. PR welcome if someone else wants to.