More gossiper cleanups that accumulated since the previous one.
* 'gleb/more-gossip-cleanup-v2' of github.com:scylladb/scylla-dev:
gossiper: replace milliseconds with seconds where appropriate
gossiper: simplify failure_detector_loop loop a bit
gossiper: use fmt library to format time
gossiper: drop on_success callback from mutate_live_and_unreachable_endpoints
gossiper: remove code duplication between shadow round and regular path when state is applied
gossiper: remove remnants of old shadow round
gossiper: fix indentation after the last patch
gossiper: co-routinize do_shadow_round
8d52f30b74