Add go.mod, build with recent Golang version

.gitlab-ci.yml

cmd/replds/server.go

debian/control

debian/rules

go.mod

go.sum

vendor/contrib.go.opencensus.io/exporter/zipkin/.travis.yml

vendor/contrib.go.opencensus.io/exporter/zipkin/LICENSE

vendor/contrib.go.opencensus.io/exporter/zipkin/Makefile

vendor/contrib.go.opencensus.io/exporter/zipkin/README.md

vendor/contrib.go.opencensus.io/exporter/zipkin/go.mod

vendor/contrib.go.opencensus.io/exporter/zipkin/go.sum

vendor/contrib.go.opencensus.io/exporter/zipkin/zipkin.go

vendor/git.autistici.org/ai3/go-common/.gitlab-ci.yml

vendor/git.autistici.org/ai3/go-common/clientutil/backend.go

...or/git.autistici.org/ai3/go-common/clientutil/balancer.go

vendor/git.autistici.org/ai3/go-common/go.mod

vendor/git.autistici.org/ai3/go-common/go.sum

vendor/git.autistici.org/ai3/go-common/renovate.json

vendor/git.autistici.org/ai3/go-common/serverutil/http.go