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As described in #13957, when creating or altering a table in group 0 mode, we don't want each node to calculate `schema::version()`s independently using a hash algorithm. Instead, we want to all nodes to use a single version for that table, commited by the group 0 command. There's even a column ready for this in `system.scylla_tables` -- `version`. This column is currently being set for system tables, but it's not being used for user tables. Similarly to what we did with global schema version in earlier commits, the obvious thing to do would be to include a live cell for the `version` column in the `system.scylla_tables` mutation when we perform the schema change in Raft mode, and to include a tombstone when performing it outside of Raft mode, for the RECOVERY case. But it's not that simple because as it turns out, we're *already* sending a `version` live cell (and also a tombstone, with timestamp decremented by 1) in all `system.scylla_tables` mutations. But then we delete that cell when doing schema merge (which begs the question why were we sending it in the first place? but I digress): ``` // We must force recalculation of schema version after the merge, since the resulting // schema may be a mix of the old and new schemas. delete_schema_version(mutation); ``` the above function removes the `version` cell from the mutation. So we need another way of distinguishing the cases of schema change originating from group 0 vs outside group 0 (e.g. RECOVERY). The method I chose is to extend `system.scylla_tables` with a boolean column, `committed_by_group0`, and extend schema mutations to set this column. In the next commit we'll decide whether or not the `version` cell should be deleted based on the value of this new column.