Add three new system tables for storing raft state for strongly
consistent tablets, corresponding to the tables for group0:
- system.raft_groups: Stores the raft log, term/vote, snapshot_id,
and commit_idx for each tablet's raft group.
- system.raft_groups_snapshots: Stores snapshot descriptors
(index, term) for each group.
- system.raft_groups_snapshot_config: Stores the raft configuration
(current and previous voters) for each snapshot.
These tables use a (shard, group_id) composite partition key with
the newly added raft_groups_partitioner and raft_groups_sharder, ensuring
data is co-located with the tablet replica that owns the raft group.
The tables are only created when the STRONGLY_CONSISTENT_TABLES experimental
feature is enabled.
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