Mux connections in parallel

This commit launches a go-routine for each accepted connection, to match
them in parallel.

cmux.go

@@ -103,6 +103,11 @@ func (m *cMux) Serve() error {
 			continue
 		}
 
+		go m.serve(c)
+	}
+}
+
+func (m *cMux) serve(c net.Conn) {
 	muc := newMuxConn(c)
 	matched := false
 outer:
@@ -121,8 +126,7 @@ func (m *cMux) Serve() error {
 		c.Close()
 		err := ErrNotMatched{c: c}
 		if !m.handleErr(err) {
-				return err
-			}
+			m.root.Close()
 		}
 	}
 }