thread/Cond: add wait() overload which takes a unique_lock<>

Just like std::condition_variable, which however has no way to specify
the std::mutex directly.

src/input/BufferedInputStream.cxx

@@ -149,7 +149,7 @@ BufferedInputStream::RunThread() noexcept
 {
 	SetThreadName("input_buffered");
 
-	const std::lock_guard<Mutex> lock(mutex);
+	std::unique_lock<Mutex> lock(mutex);
 
 	while (!stop) {
 		assert(size == buffer.size());
@@ -205,6 +205,6 @@ BufferedInputStream::RunThread() noexcept
 			client_cond.notify_one();
 			InvokeOnAvailable();
 		} else
-			wake_cond.wait(mutex);
+			wake_cond.wait(lock);
 	}
 }

src/input/Open.cxx

@@ -57,14 +57,14 @@ InputStream::OpenReady(const char *uri, Mutex &mutex)
 	is->SetHandler(&handler);
 
 	{
-		const std::lock_guard<Mutex> protect(mutex);
+		std::unique_lock<Mutex> lock(mutex);
 
 		while (true) {
 			is->Update();
 			if (is->IsReady())
 				break;
 
-			handler.cond.wait(mutex);
+			handler.cond.wait(lock);
 		}
 
 		is->Check();

src/input/ThreadInputStream.cxx

@@ -67,7 +67,7 @@ ThreadInputStream::ThreadFunc() noexcept
 {
 	FormatThreadName("input:%s", plugin);
 
-	const std::lock_guard<Mutex> lock(mutex);
+	std::unique_lock<Mutex> lock(mutex);
 
 	try {
 		Open();
@@ -85,7 +85,7 @@ ThreadInputStream::ThreadFunc() noexcept
 
 		auto w = buffer.Write();
 		if (w.empty()) {
-			wake_cond.wait(mutex);
+			wake_cond.wait(lock);
 		} else {
 			size_t nbytes;