Refactor selection logic in broadcast.go into selection.go selectors

[yondonfu]

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See the following comment from #2666

Outside of this PR I think it would be a good idea to do a refactor to move as much selection logic (i.e. last session tracking, segment in flight tracking, etc.) into MinLSSelector in selection.go to keep that logic encapsulated in a single place. This felt like a hefty change though so I opted not to do that refactor in this PR.

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I think encapsulating as much selection logic into selectors in selection.go would reduce code management in broadcast.go and allow us to just call selectors to get the next O to use without the caller needing to be exposed to how the selector is determining which O to return.

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