atomic snapshot dir, step 2 (#4093)

cmd/downloader/downloader/downloader.go

cmd/downloader/downloader/grpc_server.go

cmd/downloader/downloader/torrentcfg/torrentcfg.go

cmd/downloader/downloader/util.go

cmd/downloader/main.go

cmd/integration/commands/stages.go

cmd/utils/flags.go

eth/backend.go

eth/ethconfig/config.go

eth/stagedsync/stage_headers.go

eth/stagedsync/stage_senders_test.go

turbo/app/snapshots.go

turbo/snapshotsync/block_snapshots.go

turbo/snapshotsync/block_snapshots_test.go

turbo/stages/mock_sentry.go

turbo/stages/stageloop.go