introduce helper functions that can be used for garbage collecting old
cdc streams for tablets-based keyspaces.
- get_new_base_for_gc: finds a new base timestamp given a TTL, such that
all older timestamps and streams can be removed.
- get_cdc_stream_gc_mutations: given new base timestamp and streams,
builds mutations that update the internal cdc tables and remove the
older streams.
- garbage_collect_cdc_streams_for_table: combines the two functions
above to find a new base and build mutations to update it for a
specific table
- garbage_collect_cdc_streams: builds gc mutations for all cdc tables
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