Upgrade go-common (add tracing)

...r/git.autistici.org/ai3/go-common/clientutil/transport.go

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

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

vendor/github.com/openzipkin/zipkin-go/LICENSE

vendor/github.com/openzipkin/zipkin-go/Makefile

vendor/github.com/openzipkin/zipkin-go/README.md

vendor/github.com/openzipkin/zipkin-go/appveyor.yml

vendor/github.com/openzipkin/zipkin-go/circle.yml

vendor/github.com/openzipkin/zipkin-go/context.go

vendor/github.com/openzipkin/zipkin-go/doc.go

vendor/github.com/openzipkin/zipkin-go/endpoint.go

...ithub.com/openzipkin/zipkin-go/idgenerator/idgenerator.go

vendor/github.com/openzipkin/zipkin-go/model/annotation.go

vendor/github.com/openzipkin/zipkin-go/model/doc.go

vendor/github.com/openzipkin/zipkin-go/model/endpoint.go

vendor/github.com/openzipkin/zipkin-go/model/kind.go

vendor/github.com/openzipkin/zipkin-go/model/span.go

vendor/github.com/openzipkin/zipkin-go/model/span_id.go

vendor/github.com/openzipkin/zipkin-go/model/traceid.go

vendor/github.com/openzipkin/zipkin-go/noop.go