makes shutdown routines cleaner (#115)

packages/shutdown/order.go

packages/transactionspammer/transactionspammer.go

plugins/analysis/client/plugin.go

plugins/analysis/server/plugin.go

plugins/analysis/webinterface/httpserver/plugin.go

plugins/autopeering/plugin.go

plugins/bundleprocessor/plugin.go

plugins/dashboard/plugin.go

plugins/gossip/plugin.go

plugins/graph/plugin.go

plugins/metrics/plugin.go

plugins/statusscreen-tps/plugin.go

plugins/statusscreen/plugin.go

plugins/tangle/approvers.go

plugins/tangle/bundle.go

plugins/tangle/plugin.go

plugins/tangle/solidifier.go

plugins/tangle/transaction.go

plugins/tangle/transaction_metadata.go

plugins/ui/ui.go