[18.5] Reduce cached SQL queries in /api/v4/internal/pages endpoint

What does this MR do and why?

This backports to !209885 (merged) to 18-5-stable-ee.

Previously VirtualDomain#pages_access_control_trie dominated the CPU profile because the memoization was not working properly. It was being memoized with the value of self_and_descendants, which is a different object ID each time the method is called. Fix this by memoizing pages_access_control_trie directly.

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