RFC: Nodes are only required to publish custody columns from reconstruction

[jimmygchen]

This PR lowers the cross seeding requirements for non-supernodes, to only require publishing recovered custody columns (instead of all recovered columns) after reconstruction.

The spec currently says:

consensus-specs/specs/fulu/das-core.md

Lines 253 to 261 in 927073b

	Once the node obtains a column through reconstruction, the node MUST expose the
	new column as if it had received it over the network. If the node is subscribed
	to the subnet corresponding to the column, it MUST send the reconstructed
	`DataColumnSidecar` to its topic mesh neighbors. If instead the node is not
	subscribed to the corresponding subnet, it SHOULD still expose the availability
	of the `DataColumnSidecar` as part of the gossip emission process. After
	exposing the reconstructed `DataColumnSidecar` to the network, the node MAY
	delete the `DataColumnSidecar` if it is not part of the node's custody
	requirement.

However I think its unfair for nodes custodying less than 128 ciolumns to publish all columns instead of just its sampling columns, because they might actually end up using more outbound bandwidth than a supernode every time it performs reconstruction.

• A supernode only publishes reconstructed columns that it hasn't observed via gossip.
• A non-supernode will have to publish everything that it doesn't custody, because it wouldn't have seen any of the non sampling columns on gossip. This means a node custdoying 65 columns will always publish at least 64 (63 non custody + at least 1 missing custody column) columns every time it reconstructs.

My preference would be making publishing to non-custody subnets optional, so that non-supernodes are not required to publish the same amount of data as supernodes after reconstruction. The impact to the network should be minimal, because we can safely assume supernode exists on a live network. On a separate note, longer term it would be ideal to reduce / eliminate the dependency on supernodes via partial 1D or 2D reconstruction and partial gossip column messages.