add --datadir parameter to integration, snapshot generator, header downloader (#1749)

RELEASE_INSTRUCTIONS.md

TESTING.md

cmd/headers/README.md

cmd/headers/commands/download.go

cmd/headers/commands/root.go

cmd/integration/Readme.md

cmd/integration/commands/flags.go

cmd/integration/commands/refetence_db.go

cmd/integration/commands/reset_state.go

cmd/integration/commands/root.go

cmd/integration/commands/snapshot_check.go

cmd/integration/commands/stages.go

cmd/integration/commands/state_stages.go

cmd/rpcdaemon/commands/error_messages.go

cmd/rpcdaemon/commands/net_api.go

cmd/rpcdaemon/commands/send_transaction.go

cmd/snapshots/generator/commands/copy_from_state.go

cmd/snapshots/generator/commands/generate_body_snapshot.go

cmd/snapshots/generator/commands/generate_header_snapshot.go

cmd/snapshots/generator/commands/generate_state_snapshot.go