Modules: fix SharedDict.pop() for unexpired keys in timeout zones

Modules: fix SharedDict.pop() for unexpired keys in timeout zones

Previously, SharedDict.pop() removed an entry from the expiration rbtree
before checking whether it had expired. ngx_rbtree_delete() clears the
removed node's key, so the subsequent expiration check treated every
entry in a timeout zone as expired. pop() therefore deleted unexpired
entries but returned undefined instead of their values.

The fix preserves the expiration key before removing the node and uses
the saved value for the check in both the njs and QuickJS
implementations. Tests are added for popping an unexpired entry from a
timeout zone.