IP_ADDRESS_CACHE cache improvements

[rodpayne]

• Only add entries to the cache if the lookup was successful. (The ones that fail may work next time.)
• Retain IP_ADDRESS_CACHE entries for up to four hours. (It takes almost a day to import a day's reports, so the lookups don't need to be extremely fresh.)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 59.93%. Comparing base (041296b) to head (dc53ca2).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #501 +/- ## ========================================== + Coverage 59.84% 59.93% +0.08% ========================================== Files 12 12 Lines 1574 1575 +1 ========================================== + Hits 942 944 +2 + Misses 632 631 -1 ```

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