Use Set instead of Array for recentlyAccessedKeys

[kacper-mikolajczak]

Background

https://expensify.slack.com/archives/C05LX9D6E07/p1750073823574879?thread_ts=1749725055.638059&cid=C05LX9D6E07

Problem
recentlyAccessedKeys in OnyxCache is implemented as an Array which has O(n) read/write performance characteristics. It is a LRU but having faster access type should be nice

Solution
Convert recentlyAccessedKeys to Set so we end up with O(1) complexity.

Expected results
Improvement should be visible in the reassure performance tests deltas

[mountiny]

Ah this is in onyx repo, lets keep issues in App repo