network: handle errors and connection close more correctly

This makes writer side handle errors properly and fixes communication between
reader and writer goroutine to always correctly unregister the peer. This is
especially important for the case where error occurs before handshake
completes as in this case we don't even have goroutine in startProtocol()
running.

pkg/network/tcp_peer.go

@@ -136,7 +136,12 @@ func (p *TCPPeer) Done() chan error {
 // Disconnect will fill the peer's done channel with the given error.
 func (p *TCPPeer) Disconnect(err error) {
 	p.conn.Close()
-	p.done <- err
+	select {
+	case p.done <- err:
+		// one message to the queue
+	default:
+		// the other side may already be gone, it's OK
+	}
 }
 
 // Version implements the Peer interface.