GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/2252
Adds a Closed() method on protocol.Connection and clears up wireformatConnection a little too.
git-subtree-dir: lib/protocol git-subtree-mainline: 5ecb8bdd8a44422a59f5cbc9abedd1d1717c4247 git-subtree-split: f91191218b192ace841c878f161832d19c09145a