Take newness of bundles into account in priority function

[gardners]

Basically newly added MeshMS/MeshMB bundles should ideally gain a higher priority, so that they are pushed out quickly.

Example use case: MeshMS/MB ply of several KB total (but possibly only a few bytes newly added) will currently sit behind all smaller bundles, however old. We want instead that new bundles are treated as having a "virtual length" that is much shorter, so that they can get up-prioritised and delivered locally, before the network fully synchronises.