Kernel: Make self-contained locking smart pointers their own classes

Until now, our kernel has reimplemented a number of AK classes to
provide automatic internal locking:

- RefPtr
- NonnullRefPtr
- WeakPtr
- Weakable

This patch renames the Kernel classes so that they can coexist with
the original AK classes:

- RefPtr => LockRefPtr
- NonnullRefPtr => NonnullLockRefPtr
- WeakPtr => LockWeakPtr
- Weakable => LockWeakable

The goal here is to eventually get rid of the Lock* classes in favor of
using external locking.

Kernel/WorkQueue.h

@@ -61,7 +61,7 @@ private:
 
     void do_queue(WorkItem&);
 
-    RefPtr<Thread> m_thread;
+    LockRefPtr<Thread> m_thread;
     WaitQueue m_wait_queue;
     SpinlockProtected<IntrusiveList<&WorkItem::m_node>> m_items { LockRank::None };
 };