Add Apache Version 2.0 license and headers to all golang files

LICENSE

src/github.com/matrix-org/dendrite/clientapi/auth/auth.go

...github.com/matrix-org/dendrite/clientapi/config/config.go

....com/matrix-org/dendrite/clientapi/events/eventcontent.go

...ub.com/matrix-org/dendrite/clientapi/httputil/httputil.go

....com/matrix-org/dendrite/clientapi/jsonerror/jsonerror.go

...matrix-org/dendrite/clientapi/jsonerror/jsonerror_test.go

...com/matrix-org/dendrite/clientapi/producers/roomserver.go

...github.com/matrix-org/dendrite/clientapi/readers/login.go

...thub.com/matrix-org/dendrite/clientapi/routing/routing.go

...b.com/matrix-org/dendrite/clientapi/writers/createroom.go

...ub.com/matrix-org/dendrite/clientapi/writers/sendevent.go

...thub.com/matrix-org/dendrite/cmd/client-api-proxy/main.go

...ub.com/matrix-org/dendrite/cmd/create-room-events/main.go

...atrix-org/dendrite/cmd/dendrite-client-api-server/main.go

....com/matrix-org/dendrite/cmd/dendrite-room-server/main.go

.../matrix-org/dendrite/cmd/dendrite-sync-api-server/main.go

...github.com/matrix-org/dendrite/cmd/kafka-producer/main.go

...rix-org/dendrite/cmd/roomserver-integration-tests/main.go

src/github.com/matrix-org/dendrite/common/consumers.go