feat: [MR-730] Route `Refund` messages into streams

[alin-at-dfinity]

Fill up to half of each stream's slots (and up to ~2.5% of its target byte size) with Refunds; before routing canister messages up to 100% capacity.

This ensures fairness across refunds on the one hand; and canister messages on the other; without starving either. Refunds that do not get routed are retained in the refund pool for routing in a subsequent round.