Clean-up references to "tcollector"

[yurishkuro]

"tcollector" is the old name we used internally, for historical reasons. Many error messages in the agent still refer to it, we should clean that up.

[jpkrohling]

This is what I could find:

$ grep tcollector . -r
./cmd/agent/app/httpserver/server_test.go:			description: "baggage tcollector error",
./cmd/agent/README.md:`tcollector` service.
./cmd/agent/README.md:remote `tcollector` service. Then the server converts
./CHANGELOG.md:- Rename tcollector proxy metric in agent ([#1182](https://github.com/jaegertracing/jaeger/pull/1182), [@pavolloffay](https://github.com/pavolloffay))
./CHANGELOG.md:jaeger_http_server_errors{source="tcollector-proxy",status="5xx"}