To increase modularity, making it easier to find what is where and
maintain.
The 'log' module (cdc/log.{hh,cc}) is responsible for updating CDC log
tables when base table writes are performed.
The 'generation' module (cdc/generation.{hh,cc}) handles stream
generation changes in response to topology change events.
cdc/metadata.{hh,cc} contains a helper class which holds the currently
used generation of streams. It is used by both aforementioned modules:
'log' queries it, while 'generation' updates it.
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