Use strings for topics

[richardhuaaa]

Using PR to discuss: use string instead of bytes for topics. Batched with previous PR as both changes are disruptive on server/client and can be handled together.

Pros:

• Easier to debug
• Topics can be prefixed with human-readable labels (as opposed to reserved bytes)

Cons:

• Larger topic sizes

[richardhuaaa]

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• Use strings for topics #219 👈
• Simplify batch queries #218
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[neekolas]

I hear you on readability. But given that the topics mostly consist of 32 random bytes (inbox ID, group ID, installation ID) they're never going to be the most readable.

I think we suck it up and leave them as byte arrays with a reserved few bytes at the front to make them self-describing. It'll add up to a meaningful amount of storage and bandwidth in the long run. Especially since we have requests that might contain 10k topics.

We can always build nicer handlers in Go and Rust that take a topic, validate the format, and return a clean struct. Then no one needs to think about it again.