QuickJS: fixed potential heap-use-after-free.

QuickJS: fixed potential heap-use-after-free.

Previously in QuickJS engine, fields allocated from memory pool linked
to QuickJS engine lifetime were stored in nginx data structs.

This causes a heap-use-after-free if QuickJS engine is destroyed
earlier than a last access from nginx. For example, it becomes
visible when moving NJS cleanup handler from pool->cleanup to
r->cleanup.

The fix is to only store references in nginx objects allocated from
nginx memory pool.