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/ThreadInputStream.hxx

@@ -76,7 +76,7 @@ class ThreadInputStream : public InputStream {
 
 public:
 	ThreadInputStream(const char *_plugin,
-			  const char *_uri, Mutex &_mutex, Cond &_cond,
+			  const char *_uri, Mutex &_mutex,
 			  size_t _buffer_size) noexcept;
 
 #ifndef NDEBUG