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/storage/plugins/CurlStorage.cxx

@@ -124,9 +124,9 @@ public:
 	}
 
 	void Wait() {
-		const std::lock_guard<Mutex> lock(mutex);
+		std::unique_lock<Mutex> lock(mutex);
 		while (!done)
-			cond.wait(mutex);
+			cond.wait(lock);
 
 		if (postponed_error)
 			std::rethrow_exception(postponed_error);

src/storage/plugins/NfsStorage.cxx

@@ -167,7 +167,7 @@ private:
 	}
 
 	void WaitConnected() {
-		const std::lock_guard<Mutex> protect(mutex);
+		std::unique_lock<Mutex> lock(mutex);
 
 		while (true) {
 			switch (state) {
@@ -179,7 +179,7 @@ private:
 				}
 
 				if (state == State::INITIAL)
-					cond.wait(mutex);
+					cond.wait(lock);
 				break;
 
 			case State::CONNECTING:

src/storage/plugins/UdisksStorage.cxx

@@ -198,7 +198,7 @@ UdisksStorage::OnListReply(ODBus::Message reply) noexcept
 void
 UdisksStorage::MountWait()
 {
-	const std::lock_guard<Mutex> lock(mutex);
+	std::unique_lock<Mutex> lock(mutex);
 
 	if (mounted_storage)
 		/* already mounted */
@@ -210,7 +210,7 @@ UdisksStorage::MountWait()
 	}
 
 	while (want_mount)
-		cond.wait(mutex);
+		cond.wait(lock);
 
 	if (mount_error)
 		std::rethrow_exception(mount_error);
@@ -272,7 +272,7 @@ try {
 void
 UdisksStorage::UnmountWait()
 {
-	const std::lock_guard<Mutex> lock(mutex);
+	std::unique_lock<Mutex> lock(mutex);
 
 	if (!mounted_storage)
 		/* not mounted */
@@ -281,7 +281,7 @@ UdisksStorage::UnmountWait()
 	defer_unmount.Schedule();
 
 	while (mounted_storage)
-		cond.wait(mutex);
+		cond.wait(lock);
 
 	if (mount_error)
 		std::rethrow_exception(mount_error);