perf: call cacheKeyFn only when it is needed (if caching)

[abendi]

Previously cacheKeyFn was called unnecessarily with caching disabled as cacheKey was never used in such case. This PR makes sure cacheKeyFn is called only when caching is enabled and cacheKey is actually needed. Sometimes cacheKeyFn can use more expensive code, such as JSON.stringify, in which case it is preferred not to waste CPU cycles.

[linux-foundation-easycla]

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• ✅ login: abendi / name: Arvo Bendi (e2e7282, 965c884, 6d7fc3c, ab06a20)

[abendi]

Ping @saihaj, @ardatan 👀

[abendi]

@dimaMachina Thanks for the approval. I added a test case as well and had to fix a flow error. I don't like the usage of any in cacheMap.set((cacheKey: any), promise);. I'd prefer (cacheKey: C), but flow doesn't like that. And runtime assertion feels weird as well. Any suggestions?

[saihaj]

can you please create changeset?