Function with_possible_relaxation 

fn with_possible_relaxation<'a, SEL, OK, T>(
    selector: &'a RelaySelector<'_>,
    select: SEL,
    ok: OK,
) -> (T, SelectionInfo<'a>)
where
    SEL: FnMut(&RelaySelector<'_>) -> (T, FilterCounts),
    OK: Fn(&T) -> bool,

Re-run relay selection, relaxing our selector as necessary.

This is a helper to implement our relay selection logic. We try to run select to find one or more random relays conforming to selector. If ok says that the result is good (by returning true), we return that result. Otherwise, we try to relax the selector (if possible), and try again. If the selector can’t be relaxed any further, we return the original (not-ok) result.