Re-include missed txs into our blocks

[asonnino]

Currently, we never do it. But it might be the case that the upper layer consensus declined our block and thus we need to re-include it TXs. We need to think about the API with the consensus layer - it should tell as which blocks we can move to cold storage and which we need to retry.

We currently re-include digests until they appear in certified header. However, a certified header might still not get into the DAG. So need to think of a more accurate condition to stop re-include digests.

[LefKok]

Otherwise, add weak links in hope of including the slow blocks before GC