convert heavy into lineage (or remove it) once channel is empty (or once has blocked)

[inverted-capital]

If remove it completely, then for the purpose of making the next block, we will need to fetch past blocks. We can know which blocks we should keep around, as they will have unresolved transmissions on them. This results in the minimum current block size, and fulfills the mandate that no blocks should contain duplicated information of the previous block.