input/Stream: remove attribute "cond", replace with handler interface

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

src/input/CondHandler.hxx

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2003-2018 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * 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 2 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, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_COND_INPUT_STREAM_HANDLER_HXX
+#define MPD_COND_INPUT_STREAM_HANDLER_HXX
+
+#include "check.h"
+#include "Handler.hxx"
+#include "thread/Cond.hxx"
+
+/**
+ * An #InputStreamHandler implementation which signals a #Cond.
+ */
+struct CondInputStreamHandler final : InputStreamHandler {
+	Cond cond;
+
+	/* virtual methods from class InputStreamHandler */
+	void OnInputStreamReady() noexcept override {
+		cond.signal();
+	}
+
+	void OnInputStreamAvailable() noexcept override {
+		cond.signal();
+	}
+};
+
+#endif