LogEventWrapper.getThread only works on the first invocation

LogEventWrapper.getThread attempts to memoize the thread value but returns null if the thread field is already set where it should return the field value.

Additionally the thread is discovered using Thread.getAllStackTraces which is an expensive operation. In most cases it should be sufficient to check only the current thread, if the getThread method is called from another thread there's no guarantee that the original thread still uses the same name, or that it still exists.