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/FileInputPlugin.cxx

@@ -35,8 +35,8 @@ class FileInputStream final : public InputStream {
 
 public:
 	FileInputStream(const char *path, FileReader &&_reader, off_t _size,
-			Mutex &_mutex, Cond &_cond)
-		:InputStream(path, _mutex, _cond),
+			Mutex &_mutex)
+		:InputStream(path, _mutex),
 		 reader(std::move(_reader)) {
 		size = _size;
 		seekable = true;
@@ -54,8 +54,7 @@ public:
 };
 
 InputStreamPtr
-OpenFileInputStream(Path path,
-		    Mutex &mutex, Cond &cond)
+OpenFileInputStream(Path path, Mutex &mutex)
 {
 	FileReader reader(path);
 
@@ -75,7 +74,7 @@ OpenFileInputStream(Path path,
 
 	return std::make_unique<FileInputStream>(path.ToUTF8().c_str(),
 						 std::move(reader), info.GetSize(),
-						 mutex, cond);
+						 mutex);
 }
 
 void