Function merge_chains_up_to

fn merge_chains_up_to<D: Data>(
    chains: Vec<Chain<D>>,
    since: &Antichain<Timestamp>,
    upper: &Antichain<Timestamp>,
) -> (Chain<D>, Vec<Chain<D>>)

Merge the given chains, advancing times by the given since in the process, but only up to the given upper.

Returns the merged chain and a list of non-empty remainders of the input chains.