Currently all management of CDC generations happens in storage_service,
which is a big ball of mud that does many unrelated things.
Previous commits have introduced a new service for managing CDC
generations. This code moves most of the relevant code to this new
service.
However, some part still remains in storage_service: the bootstrap
procedure, which happens inside storage_service, must also do some
initialization regarding CDC generations, for example: on restart it
must retrieve the latest known generation timestamp from disk; on
bootstrap it must create a new generation and announce it to other
nodes. The order of these operations w.r.t the rest of the startup
procedure is important, hence the startup procedure is the only right
place for them.
Still, what remains in storage_service is a small part of the entire
CDC generation management logic; most of it has been moved to the
new service. This includes listening for generation changes and
updating the data structures for performing CDC log writes (cdc::metadata).
Furthermore these functions now return futures (and are internally
coroutines), where previously they required a seastar::async context.
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