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/output/plugins/AlsaOutputPlugin.cxx

@@ -796,7 +796,7 @@ AlsaOutput::DrainInternal()
 void
 AlsaOutput::Drain()
 {
-	const std::lock_guard<Mutex> lock(mutex);
+	std::unique_lock<Mutex> lock(mutex);
 
 	if (error)
 		std::rethrow_exception(error);
@@ -806,7 +806,7 @@ AlsaOutput::Drain()
 	Activate();
 
 	while (drain && active)
-		cond.wait(mutex);
+		cond.wait(lock);
 
 	if (error)
 		std::rethrow_exception(error);
@@ -882,7 +882,7 @@ AlsaOutput::Play(const void *chunk, size_t size)
 		   been played */
 		return size;
 
-	const std::lock_guard<Mutex> lock(mutex);
+	std::unique_lock<Mutex> lock(mutex);
 
 	while (true) {
 		if (error)
@@ -905,7 +905,7 @@ AlsaOutput::Play(const void *chunk, size_t size)
 
 		/* wait for the DispatchSockets() to make room in the
 		   ring_buffer */
-		cond.wait(mutex);
+		cond.wait(lock);
 	}
 }
 

src/output/plugins/httpd/HttpdOutputPlugin.cxx

@@ -277,9 +277,9 @@ HttpdOutput::BroadcastFromEncoder()
 {
 	/* synchronize with the IOThread */
 	{
-		const std::lock_guard<Mutex> lock(mutex);
+		std::unique_lock<Mutex> lock(mutex);
 		while (!pages.empty())
-			cond.wait(mutex);
+			cond.wait(lock);
 	}
 
 	bool empty = true;

src/output/plugins/sles/SlesOutputPlugin.cxx

@@ -317,13 +317,13 @@ SlesOutput::Play(const void *chunk, size_t size)
 		pause = false;
 	}
 
-	const std::lock_guard<Mutex> protect(mutex);
+	std::unique_lock<Mutex> lock(mutex);
 
 	assert(filled < BUFFER_SIZE);
 
 	while (n_queued == N_BUFFERS) {
 		assert(filled == 0);
-		cond.wait(mutex);
+		cond.wait(lock);
 	}
 
 	size_t nbytes = std::min(BUFFER_SIZE - filled, size);
@@ -346,12 +346,12 @@ SlesOutput::Play(const void *chunk, size_t size)
 void
 SlesOutput::Drain()
 {
-	const std::lock_guard<Mutex> protect(mutex);
+	std::unique_lock<Mutex> lock(mutex);
 
 	assert(filled < BUFFER_SIZE);
 
 	while (n_queued > 0)
-		cond.wait(mutex);
+		cond.wait(lock);
 }
 
 void