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/Scheduler.cpp

@@ -394,7 +394,7 @@ UNMAP_AFTER_INIT void Scheduler::initialize()
         current_time = current_time_monotonic;
     }
 
-    RefPtr<Thread> idle_thread;
+    LockRefPtr<Thread> idle_thread;
     g_finalizer_wait_queue = new WaitQueue;
 
     g_finalizer_has_work.store(false, AK::MemoryOrder::memory_order_release);