thread/Mutex: use std::lock_guard instead of std::unique_lock

We need purely RAII-style, and this is std::lock_guard.

src/thread/Mutex.hxx

@@ -44,7 +44,7 @@ class Mutex : public PosixMutex {};
 
 #endif
 
-using ScopeLock = std::unique_lock<Mutex>;
+using ScopeLock = std::lock_guard<Mutex>;
 
 /**
  * Within the scope of an instance, this class will keep a #Mutex