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/InputPlugin.hxx

@@ -24,7 +24,6 @@
 
 struct ConfigBlock;
 class Mutex;
-class Cond;
 class EventLoop;
 class RemoteTagScanner;
 class RemoteTagHandler;
@@ -54,8 +53,7 @@ struct InputPlugin {
 	 *
 	 * Throws std::runtime_error on error.
 	 */
-	InputStreamPtr (*open)(const char *uri,
-			       Mutex &mutex, Cond &cond);
+	InputStreamPtr (*open)(const char *uri, Mutex &mutex);
 
 	/**
 	 * Prepare a #RemoteTagScanner.  The operation must be started