High number of WebRTC peer connections is being created during transfer

Looking at the WebRTC / webtorrent part of the project, I noticed that, when downloading a file, a new offer is being created (seemingly) at every piece transferred. This happens when `TrackerClient.Announce()` is called, in which `TrackerClient.newOffer()` creates a new offer with a new WebRTC peer connection.

I'd imagined that we'd create a peer connection for each connection to a torrent peer (naming gets a bit confusing here) and keep it open for as long as we're transferring pieces from that peer, but it looks like this is not the case. I was wondering whether this is part of the torrent protocol, or if there is another reason for it?