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/AsyncInputStream.hxx

@@ -70,7 +70,7 @@ protected:
 
 public:
 	AsyncInputStream(EventLoop &event_loop, const char *_url,
-			 Mutex &_mutex, Cond &_cond,
+			 Mutex &_mutex,
 			 size_t _buffer_size,
 			 size_t _resume_at);