service: storage_service: announce new CDC generation immediately with RBNO
service: storage_service: announce new CDC generation immediately with RBNO

When a new CDC generation is created (during bootstrap or otherwise), it
is assigned a timestamp. The timestamp must be propagated as soon as
possible, so all live nodes can learn about the generation before their
clocks reach the generation's timestamp. The propagation mechanism for
generation timestamps is gossip.

When bootstrap RBNO was enabled this was not the case: the generation
timestamp was inserted into gossiper state too late, after the repair
phase finished. Fix this.

Also remove an obsolete comment.

Fixes #10149.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla