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.

test/DumpDecoderClient.hxx

@@ -23,7 +23,6 @@
 #include "check.h"
 #include "decoder/Client.hxx"
 #include "thread/Mutex.hxx"
-#include "thread/Cond.hxx"
 
 /**
  * A #DecoderClient implementation which dumps metadata to stderr and
@@ -36,7 +35,6 @@ class DumpDecoderClient final : public DecoderClient {
 
 public:
 	Mutex mutex;
-	Cond cond;
 
 	bool IsInitialized() const noexcept {
 		return initialized;