Digest: remove low seqno and compress highest_delivered and highest_received seqnos

We don't need Digest.Entry.low_seqno for NakReceiverWindow anymore. We only need it for the Retransmitter implementations. If we can remove that last dependency, e.g. by storing the last seqno removed from a retransmitter, we can remove low_seqno from a digest. This is significant, for example if we have 100 nodes, a digest includes 300 longs:(low+highest_received+highest_delivered) * 100. If we remove low_seqno, we only have 200 longs.

Also, if we could compress the highest_delivered and highest_received seqnos, we could save even more space to send digests across the network.

This is important for large clusters, to reduce the size of STABLE messages and views.