Gossiper has two blocs of endpoints, both are registered in legacy/random place in main. This PR moves them next to gossiper start and adds unregistration for both.
refs: #2737Closesscylladb/scylladb#19425
* github.com:scylladb/scylladb:
api: Remove dedicated failure_detector registration method
api: Move failure_detector endpoints set/unset to gossiper
api: Unset failure detector endpoints method
api: (Un)Register gossiper API in correct place
api: Unset gossiper endpoints on stop
asi: Coroutinize set_server_gossip()