Make controller async, add client Mixer settings

Controller uses the asio io_context and thus uses one thread less.
This also fixes a lot of thread sanitizer issues.

common/message/client_settings.hpp

@@ -0,0 +1,62 @@
+/***
+    This file is part of snapcast
+    Copyright (C) 2014-2020  Johannes Pohl
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#ifndef CLIENT_SETTINGS_H
+#define CLIENT_SETTINGS_H
+
+#include "json_message.hpp"
+
+
+namespace msg
+{
+
+class ClientSettings : public JsonMessage
+{
+public:
+    ClientSettings() : JsonMessage(message_type::kClientSettings)
+    {
+        setVolume(100);
+        setMuted(false);
+    }
+
+    ~ClientSettings() override = default;
+
+    uint16_t getVolume()
+    {
+        return get("volume", 100);
+    }
+
+    bool isMuted()
+    {
+        return get("muted", false);
+    }
+
+    void setVolume(uint16_t volume)
+    {
+        msg["volume"] = volume;
+    }
+
+    void setMuted(bool muted)
+    {
+        msg["muted"] = muted;
+    }
+};
+}
+
+
+#endif