Add WebRTC peer connection transport stats

[marcovidonis]

Looks like there is still some timing issue, whereby sometimes tc.transportStats may be null if the file transfer is too short (small file on same machine, for example). For larger file transfers, there is a high variability in the number of peer connections being created during the transfer (i.e. len(tc.transportStats)). Is this real or again a timing issue when appending new stats to the slice?

Still in draft for now.

[anacrolix]

Is it possible that connections aren't being closed, and that's why you're not seeing stats? Re the leak I mentioned (https://github.com/anacrolix/torrent/issues/763).

[marcovidonis]

We went a different direction on this, closing -- if this is of interest to anyone, let me know