Partition: add client list

For efficient traversal within one partition, e.g. for distributing
partition-local idle events.

src/Partition.hxx

@@ -32,6 +32,8 @@
 #include "Chrono.hxx"
 #include "config.h"
 
+#include <boost/intrusive/list.hpp>
+
 #include <string>
 #include <memory>
 
@@ -39,6 +41,7 @@ struct Instance;
 class MultipleOutputs;
 class SongLoader;
 class ClientListener;
+class Client;
 
 /**
  * A partition of the Music Player Daemon.  It is a separate unit with
@@ -55,6 +58,11 @@ struct Partition final : QueueListener, PlayerListener, MixerListener {
 
 	std::unique_ptr<ClientListener> listener;
 
+	boost::intrusive::list<Client,
+			       boost::intrusive::base_hook<boost::intrusive::list_base_hook<boost::intrusive::tag<Partition>,
+											    boost::intrusive::link_mode<boost::intrusive::normal_link>>>,
+			       boost::intrusive::constant_time_size<false>> clients;
+
 	MaskMonitor global_events;
 
 	struct playlist playlist;