Stopping infinite goroutines

Hey all!

Thanks for building such an awesome library!

We're getting ready to start using this as a part of our production workflow, and we wanted to build a wrapper around the `gh-ost` utility to manage some configuration on our end and to integrate it into our development/deployment workflows. We were hoping to build this wrapper in Go and consume the library directly.

When building out the proof of concept, we noticed that `migrator.go` and a few other places are starting infinite goroutines (for example the `initiateStatus`) that seem to be relying on the fact that it is invoked from a CLI and that the program will exit when `Migrate()` is complete. The behavior we saw was even after the migration completed, statuses were still being reporting to STDOUT.

The wrapper we are building will be a long running process (http server) that will manage the invocations of the gh-ost library. Would you be willing to accept a PR that cleans up these infinite goroutines, allowing the core logic to be used outside of the single CLI run?

Happy to hear any feedback on this approach if not. Thanks again!