Fix linter errors and warnings

plugins/autopeering/server/tcp/server.go

plugins/autopeering/server/udp/server.go

plugins/autopeering/types/drop/constants.go

plugins/autopeering/types/drop/drop.go

plugins/autopeering/types/peer/constants.go

plugins/autopeering/types/peer/peer.go

plugins/autopeering/types/ping/constants.go

plugins/autopeering/types/ping/ping.go

plugins/autopeering/types/request/constants.go

plugins/autopeering/types/request/request.go

plugins/autopeering/types/response/constants.go

plugins/autopeering/types/response/response.go

plugins/autopeering/types/salt/constants.go

plugins/autopeering/types/salt/salt.go

plugins/cli/plugin.go

plugins/gossip/neighbors.go

plugins/gossip/protocol_v1.go

plugins/gossip/transaction_processor_test.go

plugins/statusscreen/ui_header_bar.go

plugins/tangle/approvers.go