Address some correlation id generator issues

I noticed that in !85 (merged) I haven't wrapped the Read() method in a mutex and decided to address that. It turned you we import and use both v1 and v2 ulid package, so I dropped v1 by updating the import in tests. Also added tests for the thread-safe wrapper and enabled race detector for tests. There are three individual commits. I hope it's ok to have all of them in a single MR. Creating 3 MRs feels like an overkill here.