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
0f05fa6fd4