This feature, when enabled, will modify how schema versions
are calculated and stored.
- In group 0 mode, schema versions are persisted by the group 0 command
that performs the schema change, then reused by each node instead of
being calculated as a digest (hash) by each node independently.
- In RECOVERY mode or before Raft upgrade procedure finishes, when we
perform a schema change, we revert to the old digest-based way, taking
into account the possibility of having performed group0-mode schema
changes (that used persistent versions). As we will see in future
commits, this will be done by storing additional flags and tombstones
in system tables.
By "schema versions" we mean both the UUIDs returned from
`schema::version()` and the "global" schema version (the one we gossip
as `application_state::SCHEMA`).
For now, in this commit, the feature is always disabled. Once all
necessary code is setup in following commits, we will enable it together
with Raft.
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