mixer/Volume: refactor to class MixerMemento, per partition

Eliminate global variables, convert them to MixerMemento fields.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1583

src/Partition.hxx

@@ -25,6 +25,7 @@
 #include "queue/Listener.hxx"
 #include "output/MultipleOutputs.hxx"
 #include "mixer/Listener.hxx"
+#include "mixer/Memento.hxx"
 #include "player/Control.hxx"
 #include "player/Listener.hxx"
 #include "protocol/RangeArg.hxx"
@@ -76,6 +77,8 @@ struct Partition final : QueueListener, PlayerListener, MixerListener {
 
 	MultipleOutputs outputs;
 
+	MixerMemento mixer_memento;
+
 	PlayerControl pc;
 
 	ReplayGainMode replay_gain_mode = ReplayGainMode::OFF;