Enable frontend to ask for packets within a time frame

[tmplt]

So, the recv component of the frontend will simply copy what the backend sends it, and redraw the UI according to what the user asks. This approach will not scale. A better solution would be if the frontend queries the backend for what happens within a range of time. The backend already streams the trace to disk, we might as well revamp the implementation so it can dig up what the frontend asks for.

Originally posted by @tmplt in https://github.com/rtic-scope/cargo-rtic-trace/issues/3#issuecomment-864451516

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The backend should do all the heavy work. The less state that the frontend need keep tabs on, the better.

Will this require a database of some kind?