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/plugins/CurlInputPlugin.hxx

@@ -26,7 +26,6 @@
 #include <map>
 
 class Mutex;
-class Cond;
 
 extern const struct InputPlugin input_plugin_curl;
 
@@ -40,6 +39,6 @@ extern const struct InputPlugin input_plugin_curl;
 InputStreamPtr
 OpenCurlInputStream(const char *uri,
 		    const std::multimap<std::string, std::string> &headers,
-		    Mutex &mutex, Cond &cond);
+		    Mutex &mutex);
 
 #endif