peer: ensure goroutine launched during initial handshake exits

This commit fixes a bug which was covered by the recent server
refactoring wherein the grouting would be stuck on the send over the
message channel in the case that the handshake failed. This blockage
would create a deadlock now that the ConnectToPeer method is full
synchronous.

We fix this issue by ensuring the goroutine properly exits.

peer.go

@@ -216,6 +216,7 @@ func (p *peer) Start() error {
 		if err != nil {
 			readErr <- err
 			msgChan <- nil
+			return
 		}
 		readErr <- nil
 		msgChan <- msg