JFR: Split JFRCheckpoint VM operation

Investigating JFR crashes is a bit tedious, as Events section in hs_err shows just:

```
Event: 3.006 Executing VM operation: JFRCheckpoint
Event: 3.006 Executing VM operation: JFRCheckpoint done
```

What is that JFRCheckpoint doing is unclear, because it can do two separate things: clear or write. It would be good if we could disambiguate the two. Since there are only two flavors of checkpoint, I think we can just split the VMOp into two more precisely named ones, so it gives us e.g.:

```
Event: 2.462 Executing VM operation: JFRSafepointClear
Event: 2.463 Executing VM operation: JFRSafepointClear done
```