Add goroutine support in logrus implementation

[Deofex]

Summary

The logrus implementation always use the same Sentry hub. Logrus loggers are often shared between goroutines. Your documentation states that using separate hubs for each goroutine is mandatory.

Motivation

With the current logrus implementation a new logger is mandatory for each goroutine. It would be nice if the implementation would be able to integrate with a single logger, while still being able to use the logger in different goroutines.

[cleptric]

That makes a lot of sense, I put it into our backlog.
In the meantime, PRs are always welcome 🙂