nostrdb-net: algo for unifying similar subscriptions

[jb55]

This will be useful for our subscription manager

Example:

subA: {"authors": ["a"], "since": 100}
subB: {"authors": ["b"], "since": 200}

Should optimize to:

subAB: {"authors": ["a", "b"], "since": 100}

[v0l]

I have something like this in @snort/system-wasm but its not very well written: https://github.com/v0l/snort/blob/main/packages/system-wasm/src/merge.rs