We add garbage collection for the `CDC_GENERATIONS_V3` table to prevent
it from endlessly growing. This mechanism is especially needed because
we send the entire contents of `CDC_GENERATIONS_V3` as a part of the
group 0 snapshot.
The solution is to keep a clean-up candidate, which is one of the
already published CDC generations. The CDC generation publisher
introduced in #15281 continually uses this candidate to remove all
generations with timestamps not exceeding the candidate's and sets a new
candidate when needed.
We also add `test_cdc_generation_clearing.py` that verifies this new
mechanism.
Fixes#15323Closesscylladb/scylladb#15413
* github.com:scylladb/scylladb:
test: add test_cdc_generation_clearing
raft topology: remove obsolete CDC generations
raft topology: set CDC generation clean-up candidate
topology_coordinator: refactor publish_oldest_cdc_generation
system_keyspace: introduce decode_cdc_generation_id
system_keyspace: add cleanup_candidate to CDC_GENERATIONS_V3
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