BlockingList: fix unsafe getLockGuard() API

getLockGuard relies on copy elision to work correctly, which the compiler
is not required to do (only with C++17).
If no copy elision happens, the mutex ends up being unlocked twice, and the
CS is executed with the mutex unlocked.

The patch also ensures that the same pattern cannot be used again.

src/include/containers/BlockingList.hpp

@@ -80,7 +80,7 @@ public:
 		List<T>::clear();
 	}
 
-	LockGuard getLockGuard() { return LockGuard{_mutex}; }
+	pthread_mutex_t &mutex() { return _mutex; }
 
 private:
 

src/include/containers/LockGuard.hpp

@@ -44,6 +44,9 @@ public:
 		pthread_mutex_lock(&_mutex);
 	}
 
+	LockGuard(const LockGuard &other) = delete;
+	LockGuard &operator=(const LockGuard &other) = delete;
+
 	~LockGuard()
 	{
 		pthread_mutex_unlock(&_mutex);