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/base-foc/src/core/platform_pd.cc

@@ -128,6 +128,9 @@ Platform_pd::Platform_pd()
 
 Platform_pd::~Platform_pd()
 {
+	/* invalidate weak pointers to this object */
+	Address_space::lock_for_destruction();
+
 	for (unsigned i = 0; i < THREAD_MAX; i++) {
 		if (_threads[i])
 			_threads[i]->unbind();