performance: Fix failure to start due to lru_cache in stable
Ability to use lru_cache(user_function) was only introduced in Python 3.8. Buster has Python 3.7. This causes startup failure in Buster. Fix this by using the older API for lru_cache instead of the new one.
Tests:
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On stable and testing, starting the Plinth daemon and visiting the performance page should work.
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Adding a print statement in the _get_url() method shows that it is only called once, ever.
Signed-off-by: Sunil Mohan Adapa sunil@medhas.org