Kernel: Use a Session object to keep track of the status of a Client/Server session pair.

Reduce the associated port's connection count when a ServerSession is destroyed.

src/core/hle/kernel/session.h

@@ -0,0 +1,27 @@
+// Copyright 2017 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include "core/hle/kernel/kernel.h"
+
+namespace Kernel {
+
+class ClientSession;
+class ClientPort;
+class ServerSession;
+
+/**
+ * Parent structure to link the client and server endpoints of a session with their associated
+ * client port. The client port need not exist, as is the case for portless sessions like the
+ * FS File and Directory sessions. When one of the endpoints of a session is destroyed, its
+ * corresponding field in this structure will be set to nullptr.
+ */
+class Session final {
+public:
+    ClientSession* client = nullptr; ///< The client endpoint of the session.
+    ServerSession* server = nullptr; ///< The server endpoint of the session.
+    SharedPtr<ClientPort> port;      ///< The port that this session is associated with (optional).
+};
+}