Improve log formatting

[MMulthaupt]

1. Since some point during development logs made with Go's log package no longer get formatted correctly. The line number appears in front instead of at the end.
2. A lot of logs point to log_limiter.go as the file making the log, which is not helpful.
3. Context-cancellation as performed by reportWorkersResult() should explain why the context was cancelled.
4. Any URLs being contacted should be logged directly, and the corresponding determined proxy URL should instead only be logged if it is different.