Replace topology::transition_state::publish_cdc_generation with a queue of generations to publish and bg fiber to do it

[gleb-cloudius]

Currently topology coordinator has a topology::transition_state::publish_cdc_generation state where already created cdc generation is published to the user visible tables. This process cannot fail even if the bootstrap is aborted (otherwise some cdc updates will be missed)! With gossiper we have a fiber on each node that tries to publish the latest generation until it eventually succeed. With topology coordinator we have two options: either we hang in publish_cdc_generation state until it succeeds (this will block all other topology operations) or replace publish_cdc_generation with a static row in the topology state table that will hold not yet published generations. Instead of moving to publish_cdc_generation the generation will be added to the row. The topology coordinator will start a background fiber that will constantly try to publish all not yet published cdc generations atomically.

To publish cdc generation atomically system_distributed_keyspace::create_cdc_desc need to be changed to handle multiple generations and write multiple timestamps into cdc_generation_timestamps atomically.

[denesb]

I think CDC was not going through RAFT in 5.2 yet, so no backport needed. @kbr-scylla please re-add the label if I was wrong.