[chore] use in memory object instead of Cloudflare KV API for demo app

[baseballyama]

fix: #4264

I am accessing the Demo app from Tokyo but the response is not good.
I considered using indexedDB or Session Storage but did not adopt them because it is important that this process be performed on the back end.
And the process should not be complex to avoid making noise, so I finally opted for a simple in-memory object.
Any opinions for a better demo app?

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[gtm-nayan]

@baseballyama I thought using a Map would be more semantically correct, and abstracting the db operations would help with readability, so I've made a PR over at your branch baseballyama#6

[gtm-nayan]

The abstractions should make it easier to add some persistence as well if needed.

[baseballyama]

I don't think there is consensus on this PR yet, but I have modified it so that we can try both in-memory objects and Cloudflare KV API.
This allows us to use in-memory objects by default, but also to demonstrate using the API.
I'm not sure if we should inform users of this by some means, but I don't think it is necessary at this time.

This is a just proposal, so please close it if it is not in line with the core team's decision.