The CDC generation data can be large and not fit in a single command.
This pr splits it into multiple mutations by smartly picking a
`mutation_size_threshold` and sending each mutation as a separate group
0 command.
Commands are sent sequentially to avoid concurrency problems.
Topology snapshots contain only mutation of current CDC generation data
but don't contain any previous or future generations. If a new
generation of data is being broadcasted but hasn't been entirely applied
yet, the applied part won't be sent in a snapshot. New or delayed nodes
can never get the applied part in this scenario.
Send the entire cdc_generations_v3 table in the snapshot to resolve this
problem.
A mechanism to remove old CDC generations will be introduced as a
follow-up.
Closes#13962
* github.com:scylladb/scylladb:
test: raft topology: test `prepare_and_broadcast_cdc_generation_data`
service: raft topology: print warning in case of `raft::commit_status_unknown` exception in topology coordinator loop
raft topology: introduce `prepare_and_broadcast_cdc_generation_data`
raft: add release_guard
raft: group0_state_machine::merger take state_id as the maximal value from all merged commands
raft topology: include entire cdc_generations_v3 table in cdc_generation_mutations snapshot
raft topology: make `mutation_size_threshold` depends on `max_command_size`
raft: reduce max batch size of raft commands and raft entries
raft: add description argument to add_entry_unguarded
raft: introduce `write_mutations` command
raft: refactor `topology_change` applying
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