Thread.sleep used to be a sleepy place but there has been a bit of churn in the code recently to support virtual threads, add sleep(Duration), a JFR event, changing the underlying implementation to support sub-millis precision, and changeing the 2-arg sleep to make use of it. The changes have settled down now so we can do some cleanups that came up in PR discussions:

- Add a private sleepNanos method that creates/commits the JFR event around the sleep, this avoids duplicate code in the 3 sleep methods.
- Rename JVM_Sleep to JVM_SleepNanos to make it clear that it takes the sleep time in nanoseconds, esp. when Thread.sleep's parameter is milliseconds.