Use class dir.Rw - to separate Ro and Rw access to snapshotDir (#3534)

cmd/downloader/downloader/downloader.go

cmd/downloader/downloader/server.go

cmd/downloader/downloader/torrentcfg/torrentcfg.go

cmd/downloader/downloader/util.go

cmd/downloader/main.go

cmd/integration/commands/stages.go

cmd/rpcdaemon/cli/config.go

cmd/sentry/sentry/sentry.go

cmd/utils/flags.go

eth/backend.go

eth/ethconfig/config.go

eth/stagedsync/stage_bodies.go

eth/stagedsync/stage_headers.go

eth/stagedsync/stage_senders.go

eth/stagedsync/stage_txlookup.go

go.mod

go.sum

rules.go

turbo/app/snapshots.go

turbo/snapshotsync/block_reader.go