PeerDAS: Store only custodied columns

[nalepae]

Before this PR:

1. If a node custody < 50% of the columns, then no reconstruction happens. The node stores only columns it received via gossip.
2. I a node custody 100% of the columns, and if at least 50% of the columns are received via gossip, then 100% of the columns are reconstructed and 100% of the columns are stored.
3. If a node custody ]50%; 100%[ of the columns, and if at least 50% of the columns are received via gossip, then 100% of the columns are reconstructed and 100% of the columns are stored.

3. is kind of unexpected since some non custodied columns will be stored.

After this PR:

1. No change
2. No change
3. If a node custody ]50%; 100%[ of the columns, and if at least 50% of the columns are received via gossip, then 100% of the columns are reconstructed and only the custodied columns are stored.