base: resolve dead lock in weak pointer templates

This commit eliminates the mutual interlaced taking of destruction lock,
list lock and weak pointer locks that could lead to a dead-lock situation
when a lock pointer was tried to construct while a weak object is in
destruction progress.
Now, all weak pointers are invalidated and dequeued at the very
beginning of the weak object's destruction. Moreover, before a weak pointer
gets invalidated during destruction of a weak object, it gets dequeued, and
the list lock is freed again to avoid the former dead-lock.

Fix #1607

repos/os/src/server/nitpicker/view.h

@@ -126,6 +126,9 @@ class View : public Same_buffer_list_elem,
 
 		virtual ~View()
 		{
+			/* invalidate weak pointers to this object */
+			lock_for_destruction();
+
 			/* break link to our parent */
 			if (_parent)
 				_parent->remove_child(*this);