Implement use-after-free detection using junk and stash.

On deallocation, sampled pointers (specially aligned) get junked and stashed
into tcache (to prevent immediate reuse).  The expected behavior is to have
read-after-free corrupted and stopped by the junk-filling, while
write-after-free is checked when flushing the stashed pointers.

test/unit/tcache_max.c

@@ -152,6 +152,7 @@ TEST_BEGIN(test_tcache_max) {
 	test_skip_if(!config_stats);
 	test_skip_if(!opt_tcache);
 	test_skip_if(opt_prof);
+	test_skip_if(san_uaf_detection_enabled());
 
 	for (alloc_option = alloc_option_start;
 	     alloc_option < alloc_option_end;