Fix simple linter warnings (#562)

dapps/faucet/dapp.go

dapps/networkdelay/dapp.go

dapps/valuetransfers/packages/transaction/outputid.go

dapps/valuetransfers/packages/transaction/outputs.go

packages/gossip/events.go

plugins/analysis/client/plugin.go

plugins/analysis/packet/fpc_heartbeat.go

plugins/analysis/packet/heartbeat.go

plugins/autopeering/plugin.go

plugins/banner/plugin.go

plugins/bootstrap/plugin.go

plugins/cli/plugin.go

plugins/config/plugin.go

plugins/dashboard/plugin.go

plugins/drng/plugin.go

plugins/gossip/plugin.go

plugins/gracefulshutdown/plugin.go

plugins/issuer/plugin.go

plugins/logger/plugin.go

plugins/portcheck/plugin.go